This change extends the peer metrics collection:

- traces the life-cycle of the peers
- meters the peer traffic separately for every peer
- creates event feed for the peer events
- emits the peer events

This PR adds enode.LocalNode and integrates it into the p2p
subsystem. This new object is the keeper of the local node
record. For now, a new version of the record is produced every
time the client restarts. We'll make it smarter to avoid that in
the future.

There are a couple of other changes in this commit: discovery now
waits for all of its goroutines at shutdown and the p2p server
now closes the node database after discovery has shut down. This
fixes a leveldb crash in tests. p2p server startup is faster
because it doesn't need to wait for the external IP query
anymore.

Package p2p/enode provides a generalized representation of p2p nodes
which can contain arbitrary information in key/value pairs. It is also
the new home for the node database. The "v4" identity scheme is also
moved here from p2p/enr to remove the dependency on Ethereum crypto from
that package.

Record signature handling is changed significantly. The identity scheme
registry is removed and acceptable schemes must be passed to any method
that needs identity. This means records must now be validated explicitly
after decoding.

The enode API is designed to make signature handling easy and safe: most
APIs around the codebase work with enode.Node, which is a wrapper around
a valid record. Going from enr.Record to enode.Node requires a valid
signature.

* p2p/discover: port to p2p/enode

This ports the discovery code to the new node representation in
p2p/enode. The wire protocol is unchanged, this can be considered a
refactoring change. The Kademlia table can now deal with nodes using an
arbitrary identity scheme. This requires a few incompatible API changes:

  - Table.Lookup is not available anymore. It used to take a public key
    as argument because v4 protocol requires one. Its replacement is
    LookupRandom.
  - Table.Resolve takes *enode.Node instead of NodeID. This is also for
    v4 protocol compatibility because nodes cannot be looked up by ID
    alone.
  - Types Node and NodeID are gone. Further commits in the series will be
    fixes all over the the codebase to deal with those removals.

* p2p: port to p2p/enode and discovery changes

This adapts package p2p to the changes in p2p/discover. All uses of
discover.Node and discover.NodeID are replaced by their equivalents from
p2p/enode.

New API is added to retrieve the enode.Node instance of a peer. The
behavior of Server.Self with discovery disabled is improved. It now
tries much harder to report a working IP address, falling back to
127.0.0.1 if no suitable address can be determined through other means.
These changes were needed for tests of other packages later in the
series.

* p2p/simulations, p2p/testing: port to p2p/enode

No surprises here, mostly replacements of discover.Node, discover.NodeID
with their new equivalents. The 'interesting' API changes are:

 - testing.ProtocolSession tracks complete nodes, not just their IDs.
 - adapters.NodeConfig has a new method to create a complete node.

These changes were needed to make swarm tests work.

Note that the NodeID change makes the code incompatible with old
simulation snapshots.

* whisper/whisperv5, whisper/whisperv6: port to p2p/enode

This port was easy because whisper uses []byte for node IDs and
URL strings in the API.

* eth: port to p2p/enode

Again, easy to port because eth uses strings for node IDs and doesn't
care about node information in any way.

* les: port to p2p/enode

Apart from replacing discover.NodeID with enode.ID, most changes are in
the server pool code. It now deals with complete nodes instead
of (Pubkey, IP, Port) triples. The database format is unchanged for now,
but we should probably change it to use the node database later.

* node: port to p2p/enode

This change simply replaces discover.Node and discover.NodeID with their
new equivalents.

* swarm/network: port to p2p/enode

Swarm has its own node address representation, BzzAddr, containing both
an overlay address (the hash of a secp256k1 public key) and an underlay
address (enode:// URL).

There are no changes to the BzzAddr format in this commit, but certain
operations such as creating a BzzAddr from a node ID are now impossible
because node IDs aren't public keys anymore.

Most swarm-related changes in the series remove uses of
NewAddrFromNodeID, replacing it with NewAddr which takes a complete node
as argument. ToOverlayAddr is removed because we can just use the node
ID directly.

This change removes a peer information from dialing history
when peer is removed from static list. It allows to force a
server to re-dial concrete peer if it is needed.

In our case we are running geth node on mobile devices, and
it is common for a network connection to flap on mobile.
Almost every time it flaps or network connection is changed
from cellular to wifi peers are disconnected with read
timeout. And usually it takes 30 seconds (default expiration
timeout) to recover connection with static peers after
connectivity is restored.

This change allows us to reconnect with peers almost
immediately and it seems harmless enough.

p2p/simulations: introduce dialBan

- Refactor simulations/network connection getters to support
  avoiding simultaneous dials between two peers If two peers dial
  simultaneously, the connection will be dropped to help avoid
  that, we essentially lock the connection object with a
  timestamp which serves as a ban on dialing for a period of time
  (dialBanTimeout).

- The connection getter InitConn can be wrapped and passed to the
  nodes via adapters.NodeConfig#Reachable field and then used by
  the respective services when they initiate connections. This
  massively stablise the emerging connectivity when running with
  hundreds of nodes bootstrapping a network.

p2p: add Inbound public method to p2p.Peer

p2p/simulations: Add server id to logs to support debugging
in-memory network simulations when multiple peers are logging.

p2p: SetupConn now returns error. The dialer checks the error and
only calls resolve if the actual TCP dial fails.

This commit introduces a network simulation framework which
can be used to run simulated networks of devp2p nodes. The
intention is to use this for testing protocols, performing
benchmarks and visualising emergent network behaviour.

The p2p packages can now be configured to restrict all communication to
a certain subset of IP networks. This feature is meant to be used for
private networks.

This change makes it possible to add peers without providing their IP
address. The endpoint of the target node is resolved using the discovery
protocol.

This change simplifies the dial scheduling logic because it
no longer needs to track whether the discovery table has been
bootstrapped.

This reverts commit 5c949d3b.

Package fdtrack logs statistics about open file descriptors.
This should help identify the source of #1549.

I forgot to update one instance of "go-ethereum" in commit 3f047be5.

All code outside of cmd/ is licensed as LGPL. The headers
now reflect this by calling the whole work "the go-ethereum library".

Lookup calls would spin out of control when network connectivity was
lost. The throttling that was in place only took effect when the table
returned zero results, which doesn't happen very often.

The new throttling should not have a negative impact when the host is
online. Lookups against the network take some time and dials for all
results must complete or hit the cache before a new one is started. This
usually takes longer than four seconds, leaving online lookups
unaffected.

Fixes #1296

The most visible change is event-based dialing, which should be an
improvement over the timer-based system that we have at the moment.
The dialer gets a chance to compute new tasks whenever peers change or
dials complete. This is better than checking peers on a timer because
dials happen faster. The dialer can now make more precise decisions
about whom to dial based on the peer set and we can test those
decisions without actually opening any sockets.

Peer management is easier to test because the tests can inject
connections at checkpoints (after enc handshake, after protocol
handshake).

Most of the handshake stuff is now part of the RLPx code. It could be
exported or move to its own package because it is no longer entangled
with Server logic.