wallet: track mempool conflicts

Behavior changes are:
- if a tx has a mempool conflict, the wallet will not attempt to
  rebroadcast it
- if a txo is spent by a mempool-conflicted tx, that txo is no
  longer considered spent

test/functional/wallet_abandonconflict.py

@@ -232,7 +232,11 @@ class AbandonConflictTest(BitcoinTestFramework):
         balance = newbalance
 
         # Invalidate the block with the double spend. B & C's 10 BTC outputs should no longer be available
-        self.nodes[0].invalidateblock(self.nodes[0].getbestblockhash())
+        blk = self.nodes[0].getbestblockhash()
+        # mine 10 blocks so that when the blk is invalidated, the transactions are not
+        # returned to the mempool
+        self.generate(self.nodes[1], 10)
+        self.nodes[0].invalidateblock(blk)
         assert_equal(alice.gettransaction(txAB1)["confirmations"], 0)
         newbalance = alice.getbalance()
         assert_equal(newbalance, balance - Decimal("20"))