Add mempool_fee_delta functional test

This MR is related to Bitcoin-ABC#290.

The new functional test sets fee delta (positive and negative)
and checks that on reorg the mempool tx priorities are preserved.

This exercises the DisconnectedBlockTransactions class on the C++
side and ensures that it keeps the right metadata when it takes its
importMempool path on reorg.

Test plan:
- `test/functional/test_runner.py mempool_fee_delta`

test/functional/mempool_fee_delta.py

@@ -0,0 +1,82 @@
+#!/usr/bin/env python3
+# Copyright (c) 2021 The Bitcoin developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test mempool feeDelta both positive and negative, then reorgs"""
+
+from decimal import Decimal, getcontext
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal,
+    sync_blocks,
+    sync_mempools,
+    connect_nodes,
+    disconnect_nodes
+)
+
+
+class MempoolFeeDeltaTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 4
+        getcontext().prec = 8
+        self.extra_args = [
+            [
+                '-blockmintxfee=0.000001',
+            ],
+            [], [], []
+        ]
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_wallet()
+
+    def create_prioritized_transaction(self, node, address, fee_delta):
+        txid = node.sendtoaddress(address, 1)
+        tx_initialfee = node.getmempoolentry(txid)['fees']['modified']
+        node.prioritisetransaction(txid=txid, fee_delta=fee_delta)
+        tx_modifiedfee = node.getmempoolentry(txid)['fees']['modified']
+        return txid, tx_initialfee, tx_modifiedfee
+
+    def run_test(self):
+
+        address = self.nodes[0].getnewaddress()
+        disconnect_nodes(self.nodes[1], self.nodes[2])
+
+        # Create a transaction that will be never added in a block
+        txid0, tx0_initialfee, tx0_modifiedfee = self.create_prioritized_transaction(
+            self.nodes[0], address, -220)
+
+        # Create 1 block for the first group of nodes to make reorg code kick in
+        # below.
+        self.nodes[0].generate(1)
+        sync_blocks(self.nodes[0:1])
+        sync_mempools(self.nodes[0:1])
+
+        # check txid0 is still in mempool
+        assert self.nodes[0].getmempoolentry(txid0)
+
+        # Create a transaction and prioritise it with negative fee delta
+        txid1, tx1_initialfee, tx1_modifiedfee = self.create_prioritized_transaction(
+            self.nodes[0], address, -1)
+        assert_equal(tx1_modifiedfee, tx1_initialfee - Decimal(0.00000001))
+
+        # Create a transaction and prioritise it with positive fee delta
+        txid2, tx2_initialfee, tx2_modifiedfee = self.create_prioritized_transaction(
+            self.nodes[0], address, 2000)
+        assert_equal(tx2_modifiedfee, tx2_initialfee + Decimal(0.00002000))
+
+        # Trigger reorg -- block created above will roll back so the DisconnectedBlockTransactions
+        # code on the C++ side will kick in, temporarily emptying out the mempool and filling it
+        # again. Fee deltas for in-memory txs should be preserved across this process.
+        self.nodes[3].generate(10)
+        connect_nodes(self.nodes[1], self.nodes[2])
+        sync_blocks(self.nodes)
+
+        tx1_postreorgfee = self.nodes[0].getmempoolentry(txid1)['fees']['modified']
+        tx2_postreorgfee = self.nodes[0].getmempoolentry(txid2)['fees']['modified']
+
+        assert_equal(tx1_modifiedfee, tx1_postreorgfee)
+        assert_equal(tx2_modifiedfee, tx2_postreorgfee)
+
+if __name__ == '__main__':
+    MempoolFeeDeltaTest().main()