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.

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Get block hash given a block height - A `get_block_hash` method is now defined on the `GetBlockHash` trait and implemented on every blockchain backend. This method expects a block height and returns the corresponding block hash. 
 - Add `remove_partial_sigs` and `try_finalize` to `SignOptions`
 - Deprecate `AddressValidator`
+- Fix Electrum wallet sync potentially causing address index decrement - compare proposed index and current index before applying batch operations during sync.
 
 ## [v0.19.0] - [v0.18.0]