Fix: Wallet sync may decrement address index

Fix: Wallet sync may decrement address index

This bug seems to be Electrum-specific. The fix is to check the
proposed changes against the current state of the database. Ensure
newly suggested indexes are not smaller than indexes already in
database.

Changes:
* Check index updates before they are applied to database during
  Electrum Blockchain sync (Thank you @rajarshimaitra for providing
  an elegant solution).

Tests added:
* bdk_blockchain_tests!::test_sync_address_index_should_not_decrement
* bdk_blockchain_tests!::test_sync_address_index_should_increment

These tests ensure there will be no unexpected address reuse when
grabbing a new address via `Wallet::get_address` with `AddressIndex::New`.

Other changes:
* Tweak `rpc.rs` so that clippy is happy.