rpc: Return total fee in mempool

Also, add missing lock annotations

test/functional/mempool_persist.py

@@ -69,6 +69,8 @@ class MempoolPersistTest(BitcoinTestFramework):
         assert_equal(len(self.nodes[0].getrawmempool()), 5)
         assert_equal(len(self.nodes[1].getrawmempool()), 5)
 
+        total_fee_old = self.nodes[0].getmempoolinfo()['total_fee']
+
         self.log.debug("Prioritize a transaction on node0")
         fees = self.nodes[0].getmempoolentry(txid=last_txid)['fees']
         assert_equal(fees['base'], fees['modified'])
@@ -76,6 +78,10 @@ class MempoolPersistTest(BitcoinTestFramework):
         fees = self.nodes[0].getmempoolentry(txid=last_txid)['fees']
         assert_equal(fees['base'] + Decimal('0.00001000'), fees['modified'])
 
+        self.log.info('Check the total base fee is unchanged after prioritisetransaction')
+        assert_equal(total_fee_old, self.nodes[0].getmempoolinfo()['total_fee'])
+        assert_equal(total_fee_old, sum(v['fees']['base'] for k, v in self.nodes[0].getrawmempool(verbose=True).items()))
+
         tx_creation_time = self.nodes[0].getmempoolentry(txid=last_txid)['time']
         assert_greater_than_or_equal(tx_creation_time, tx_creation_time_lower)
         assert_greater_than_or_equal(tx_creation_time_higher, tx_creation_time)