Implement transaction pool and relay.

This commit is a rather large one which implements transaction pool and
relay according to the protocol rules of the reference implementation.
It makes use of btcchain to ensure the transactions are valid for the
block chain and includes several stricter checks which determine if they
are "standard" or not before admitting them into the pool and relaying
them.

There are still a few TODOs around the more strict rules which determine
which transactions are willing to be mined, but the core checks which
are imperative (everything except the all of the "standard" checks really)
to operate as a good citizen on the bitcoin network are in place.