Move transaction handling into block manager.

The block manager handles inventory messges to know which inventory should
be requested based on what is already known and what is already in flight.
So, this commit adds logic to ask the transaction memory pool if the
transaction is already known before requesting it and tracks pending
requests into an in-flight transaction map owned by the block manager.

It also moves the transaction processing into the block manager so the
in-flight map can be properly cleaned.