Two queues for high-priority and ordinary messages. Fixes#590. These queues
are deliberately made small to avoid buffer bloat problem, there is gonna be
another queueing layer above them to compensate for that. The queues are
designed to be synchronous in enqueueing, async capabilities are to be added
layer above later.
Otherwise the node might crash in `startProtocol` because of missing Version
field in the peer. And it also keeps the sequence correct, Version MUST be
sent first and ACKs can only follow it.
This allows to start handshaking from both client and server (mainnet/testnet
nodes were seen to not care about string ordering for it), but still maintains
some sane checks in the process. It also makes functions thread-safe because
we have two goroutines servicing read and write side of the Peer connection,
so they can clash on access to the struct fields.
Add a test for it also.
