Drop fee tx hex once the fee is confirmed

[jholdstock]

Fee tx hex makes up a huge amount of the database size. Once the fee tx is broadcast and has 6 confirmations, keeping the hex in the database is unnecessary so long as we keep the hash.

I propose a database migration to clear out any old fee tx hex which are no longer required, and a change to background.go which will ensure that any new fee tx hex are removed once they are confirmed.

[jholdstock]

Closed by #260