keychain_txout: `apply_changeset` should restore cache before applying last revealed

[ValuedMammal]

In order to reap the most benefit from the SPK cache, it seems necessary to refill the cache before applying the changes to last_revealed, however we currently do things in the opposite order. I noticed this while running performance tests locally.

[ValuedMammal]

So far I haven't seen a huge difference when testing against the current benchmark in #1968, maybe it got optimized by the compiler. But I did observe a measurable difference using the spk cache test on this branch https://github.com/matthiasdebernardini/bdk-sqlx/tree/bdkwallet2 modified for my environment.