Tpool Fast Start

To support the transaction pool's lack of need to actually be aware of any consensus history, I have changed the ConsensusSetSubscribe function to take a few special cases, one of which won't give you any consensus changes, but subscribe you to the beginning. Meaning you may have missed a bunch of updates, but you may not care (as is the case with the transaction pool)

The diff is pretty big because I touched up a few other things as well.