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.

src/blockchain/rpc.rs

@@ -340,7 +340,7 @@ impl WalletSync for RpcBlockchain {
                     ),
                     received,
                     sent,
-                    fee: tx_result.fee.map(|f| f.as_sat().abs() as u64),
+                    fee: tx_result.fee.map(|f| f.as_sat().unsigned_abs()),
                 };
                 debug!(
                     "saving tx: {} tx_result.fee:{:?} td.fees:{:?}",