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:{:?}",

src/blockchain/script_sync.rs

@@ -314,6 +314,22 @@ impl<'a, D: BatchDatabase> State<'a, D> {
         let finished_txs = make_txs_consistent(&self.finished_txs);
         let observed_txids: HashSet<Txid> = finished_txs.iter().map(|tx| tx.txid).collect();
         let txids_to_delete = existing_txids.difference(&observed_txids);
+
+        // Ensure `last_active_index` does not decrement database's current state.
+        let index_updates = self
+            .last_active_index
+            .iter()
+            .map(|(keychain, sync_index)| {
+                let sync_index = *sync_index as u32;
+                let index_res = match self.db.get_last_index(*keychain) {
+                    Ok(Some(db_index)) => Ok(std::cmp::max(db_index, sync_index)),
+                    Ok(None) => Ok(sync_index),
+                    Err(err) => Err(err),
+                };
+                index_res.map(|index| (*keychain, index))
+            })
+            .collect::<Result<Vec<(KeychainKind, u32)>, _>>()?;
+
         let mut batch = self.db.begin_batch();
 
         // Delete old txs that no longer exist
@@ -377,8 +393,10 @@ impl<'a, D: BatchDatabase> State<'a, D> {
             batch.set_tx(finished_tx)?;
         }
 
-        for (keychain, last_active_index) in self.last_active_index {
-            batch.set_last_index(keychain, last_active_index as u32)?;
+        // apply index updates
+        for (keychain, new_index) in index_updates {
+            debug!("updating index ({}, {})", keychain.as_byte(), new_index);
+            batch.set_last_index(keychain, new_index)?;
         }
 
         info!(