Merge bitcoindevkit/bdk#1993: keychain_txout: `apply_changeset` restores spk cache...

Merge bitcoindevkit/bdk#1993: keychain_txout: `apply_changeset` restores spk cache before last revealed

6ddecc983ab3fb163303af4902bd10cf7e4a1fdf refactor(keychain_txout): `apply_changeset` restores spk cache before last revealed (valued mammal)

Pull request description:

  This patch improves readability and maintains logical consistency with the use of `spk_cache` throughout the `keychain_txout` module.

  While it might offer a performance benefit, the results are mostly comparable with the current benchmarks as far as I can tell. At least there's no indication that it would negatively impact performance.

  fixes #1975

  ### Changelog notice

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

ACKs for top commit:
  LagginTimes:
    ACK 6ddecc983ab3fb163303af4902bd10cf7e4a1fdf

Tree-SHA512: 4c11d049f94f332c88c63687d5147d25cc372d65b5f1d9d9c8784e1310e0f03e9aee479b684c255dbebb1510a7922331ad07cbcbdf47d534cf28ea8ee0e1be66