Merge bitcoindevkit/bdk#653: Fix: Wallet sync may decrement address index

af6bde3997ac2fe4cfdbb691f2af2e802f0ab7c6 Fix: Wallet sync may decrement address index (志宇)

Pull request description:

  ### Description

  Fixes #649

  It is critical to ensure `Wallet::get_address` with `AddressIndex::new` always returns a new and unused address.

  This bug seems to be Electrum-specific. The fix is to check address index updates to  ensure that newly suggested indexes are not smaller than indexes already in database.

  ### Notes to the reviewers

  I have written new tests in `/testutils/blockchain_tests.rs` that tests all `Blockchain` implementations.

  ### 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

  #### Bugfixes:

  ~* [ ] This pull request breaks the existing API~
  * [x] I've added tests to reproduce the issue which are now passing
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  afilini:
    ACK af6bde3997ac2fe4cfdbb691f2af2e802f0ab7c6

Tree-SHA512: d714bebcf7c2836f8b98129b39b4939b0e36726acf0208e52d501f433be6cdb12f1abebc28bd7da0be8b780ccce6e1e42c8fdc6633dd486bf329bc6f88e1ce67