Merge bitcoindevkit/bdk#1109: Further improve unconfirmed tx conflict resolution

afbf83c8b0d5c97495e6ceaa39963415edef6911 chain(fix): conflict resolution for txs with same last_seen (Wei Chen)

Pull request description:

  ### Description

  Fixes #1102. If a conflicting tx has the same `last_seen`, then we check lexicographical sorting of txids.

  ### Notes to the reviewers

  The tests for this fix exist in the `TxTemplate` structure in #1064 which may need to be merged first.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  danielabrozzoni:
    ACK afbf83c8b0d5c97495e6ceaa39963415edef6911

Tree-SHA512: 91b8fbff305b715247501b861ab7ea9e9d9ef99248b05d14e01aacf7e64ad7826f35773e8998cf421dbd04f663714026084c6e817ac5365bce4844c8ea3b7e3f