p2p: more concurrency fixups

During a split parent and grand parent were included in the database but
not in the canonical chain (numbered chain). Added a `merge` function
which finds the common ancestor of the chains and reinserts the missing
blocks.

Fix links

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

Processing transactions should continue when a transaction throws a gas
limit reached error. Other transactions may actually succeed.

Rlock => Lock when creating a new nonce

removePeer can be called even after listenLoop and dialLoop have returned.

blockpool stability fixes:

Added blockchain DB versioning support, closes #650

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.

This regression was introduced in b3c058a9.

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.