Where 404s before - now linking to the path that they stay valid a little longer ;-)

blockpool stability fixes:

Added blockchain DB versioning support, closes #650

Add transaction tests

When a parent is missing in the block list an attempt should be made to
fetch the missing parent and grandparents.

- follow up locks and fix them
- chainManager: call SetQueued for parentErr future blocks, uncomment TD checks, unskip test
- make ErrIncorrectTD non-fatal to be forgiving to genuine mistaken nodes (temp) but demote them to guard against stuck best peers.
- add purging to bounded nodeCache (config nodeCacheSize)
- use nodeCache when creating blockpool entries and let non-best peers add blocks (performance boost)
- minor error in addError
- reduce idleBestPeerTimeout to 1 minute
- correct status counts and unskip status passing status test
- glogified logging

Whisper cleanup, part 1

If a peer was somehow already fetching and somehow managed to end up in
the `available` pool it should return it's work.

bugfixes for headsection deadlocks

- switchC initialised as closed
- move select in setChainInfoFromBlock out of peer lock

td update from node + bugfix

p2p: a bunch of fixes

- reorg and simplify AddBlock
- introduce nodeCache
- TestPeerPromotionByTdOnBlock unskipped and passes
- move switchC/idleC channel creation around: solves deadlock (now respects the contract with section process: either can activate or complete at any one time)

This is supposed to apply some back pressure so Server is not accepting
more connections than it can actually handle. The current limit is 50.
This doesn't really need to be configurable, but we'll see how it
behaves in our test nodes and adjust accordingly.

As of this commit, p2p will disconnect nodes directly after the
encryption handshake if too many peer connections are active.
Errors in the protocol handshake packet are now handled more politely
by sending a disconnect packet before closing the connection.

Bugs fixed:
  - Use randomly generated flags as the spec required.
  - During envelope opening check the first bit only for signature.

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 commit introduces a new (temporary) peer selection
strategy based on random lookups.

While we're here, also implement the TODOs in dialLoop.