Fix Transaction Pool Bug

The transaction pool was previously experiencing a bug where transaction sets with parents that had been accepted to the blockchain would be rejected as double-spends. This was preventing hosts from getting paid. The transaction pool now keeps a record of all the transactions that have been confirmed on the blockchain, and will remove the parents from a transaction set that have been confirmed.

This behavior should be tested in an rc, but I think this PR is the last code change we need before v0.6.0, the rest should just be adjusting the constants.