Revert "wallet: Remove in-memory utxoset filter"

This reverts commit c239a71.

The goal of c239a71 was to reduce the memory footprint of our
internal UTXO set tracking, and testing against the sqlite3 backend
showed no performance impact. We have since found that the added
roundtrips to the DB server with postgres was having a considerable
performance impact, and backups would also bloat due to the increased
number of queries.

Undoing this change skips the noop updates that were causing this
regression.

Changelog-Fixed: db: Fixed a performance regression during block sync, resulting in many more queries against the DB than necessary.