Using a Timer over Ticker seems to be a lot better, though I cannot fully
account for why that it behaves so (since Ticker should be more bursty, but not
necessarily more active over time, but that may depend on how long window it
uses to decide on when to tick next)

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".

As of this commit, we no longer rely on the protocol handler to report
write errors in a timely fashion. When a write fails, shutdown is
initiated immediately and no new writes can start. This will also
prevent new writes from starting after Server.Stop has been called.

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.

The previous limit was 10MB which is unacceptable for all kinds
of reasons, the most important one being that we don't want to
allow the remote side to make us allocate 10MB at handshake time.

The returned reason is currently not used except for the log
message. This change makes the log messages a bit more useful.
The handshake code also returns the remote reason.

p2p.Msg.ReceivedAt can be used for determining block propagation from
begining to end.

Peer.readLoop will only terminate if the connection is closed. Fix the
hang by closing the connection before waiting for readLoop to terminate.

This also removes the british disconnect procedure where we're waiting
for the remote end to close the connection. I have confirmed with
@subtly that cpp-ethereum doesn't adhere to it either.

netWrapper already sets a read deadline in ReadMsg.

There were multiple synchronization issues in the disconnect handling,
all caused by the odd special-casing of Peer.readLoop errors. Remove the
special handling of read errors and make readLoop part of the Peer
WaitGroup.

Thanks to @Gustav-Simonsson for pointing at arrows in a diagram
and playing rubber-duck.

This helps a lot with debugging.

Message encoding functions have been renamed to catch any uses.
The switch to the new encoder can cause subtle incompatibilities.
If there are any users outside of our tree, they will at least be
alerted that there was a change.

NewMsg no longer exists. The replacements for EncodeMsg are called
Send and SendItems.

They got lost in the transition to rlpxFrameRW.

With RLPx frames, the message code is contained in the
frame and is no longer part of the encoded data.

EncodeMsg, Msg.Decode have been updated to match.
Code that decodes RLP directly from Msg.Payload will need
to change.

This should prevent connection drops.

The diff is a bit bigger than expected because the protocol handshake
logic has moved out of Peer. This is necessary because the protocol
handshake will have custom framing in the final protocol.

For compatibility with cpp-ethereum

There are now two deadlines, frameReadTimeout and payloadReadTimeout.

The frame timeout is longer and allows for connections that are idle.
The message timeout is still short and ensures that we don't get stuck
in the middle of a message.

The unit test hooks were turned on 'in production'.

Overview of changes:

- ClientIdentity has been removed, use discover.NodeID
- Server now requires a private key to be set (instead of public key)
- Server performs the encryption handshake before launching Peer
- Dial logic takes peers from discover table
- Encryption handshake code has been cleaned up a bit
- baseProtocol is gone because we don't exchange peers anymore
- Some parts of baseProtocol have moved into Peer instead

- add const length params for handshake messages
- add length check to fail early
- add debug logs to help interop testing (!ABSOLUTELY SHOULD BE DELETED LATER)
- wrap connection read/writes in error check
- add cryptoReady channel in peer to signal when secure session setup is finished
- wait for cryptoReady or timeout in TestPeersHandshake