This PR introduces a database migration which deletes any raw fee transactions which remain in the database after already having been confirmed on-chain. There is no need to keep these, and they take up a lot of space.
There is also a change in the background handler to ensure that in future, tx hex is always removed from the database when the tx is confirmed.
Plus a few other miscellaneous pieces which will be usedful soon:
- Remove `Get` from func names `GetCookieSecret` and `GetFeeXPub`.
- Add helpers to encode/decode integers/bytes.