Add transaction tests

Whisper cleanup, part 1

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.

* Add initial go wrapping for TransactionTests with some tests
  disabled in lieu of consistent HEX encodings and a few other
  pending bugfixes
* TODO: Consider better way of perhaps modelling each test in
  the JSON files as a single Go test, instead of one Go test per
  JSON file

xeth: fix #640, panic converting nil recipient to hex.

Fetching the recipient address from a transaction was changed to return nil
instead of a zero-address, but this code path was not updated, so whenever
a contract was created, a nil panic occured.

bugfix as a result of PR 671