Merge bitcoindevkit/bdk#611: Discourage fee sniping with nLockTime

97bc9dc7170c336e97cd756b1f07ac3c23a39626 Discourage fee sniping with nLockTime (Daniela Brozzoni)

Pull request description:

  ### Description
  By default bdk sets the transaction's nLockTime to current_height
  to prevent fee sniping.
  current_height can be provided by the user through TxParams; if the user
  didn't provide it, we use the last sync height, or 0 if we never synced.

  Fixes https://github.com/bitcoindevkit/bdk/issues/533

  ### Notes to the reviewers:

  If you want to know more about fee sniping: https://bitcoinops.org/en/topics/fee-sniping/

  ### 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

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] I've added docs for the new feature
  * [x] I've updated `CHANGELOG.md`

ACKs for top commit:
  afilini:
    ACK 97bc9dc7170c336e97cd756b1f07ac3c23a39626

Tree-SHA512: e92d1ae907687d9fee44d120d790f1ebdf14b698194979e1be8433310fd5636afa63808effed12fce6091f968ec6b76b727cfee6fed54068af0a7450239fdd26