tests: Use batched RPC in feature_fee_estimation

feature_fee_estimation has a lot of loops that hit the RPC many times in
succession in order to setup scenarios. Using batched requests for these
can reduce the test's runtime without effecting the test's behavior.

test/functional/test_framework/wallet.py

@@ -141,6 +141,10 @@ class MiniWallet:
             if out['scriptPubKey']['hex'] == self._scriptPubKey.hex():
                 self._utxos.append(self._create_utxo(txid=tx["txid"], vout=out["n"], value=out["value"], height=0))
 
+    def scan_txs(self, txs):
+        for tx in txs:
+            self.scan_tx(tx)
+
     def sign_tx(self, tx, fixed_length=True):
         """Sign tx that has been created by MiniWallet in P2PK mode"""
         assert_equal(self._mode, MiniWalletMode.RAW_P2PK)