RPC: indicate which transactions are replaceable

Add "bip125-replaceable" output field to listtransactions and gettransaction
which indicates if an unconfirmed transaction, or any unconfirmed parent, is
signaling opt-in RBF according to BIP 125.

src/policy/rbf.h

@@ -0,0 +1,20 @@
+// Copyright (c) 2016 The Bitcoin developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_POLICY_RBF_H
+#define BITCOIN_POLICY_RBF_H
+
+#include "txmempool.h"
+
+// Check whether the sequence numbers on this transaction are signaling
+// opt-in to replace-by-fee, according to BIP 125
+bool SignalsOptInRBF(const CTransaction &tx);
+
+// Determine whether an in-mempool transaction is signaling opt-in to RBF
+// according to BIP 125
+// This involves checking sequence numbers of the transaction, as well
+// as the sequence numbers of all in-mempool ancestors.
+bool IsRBFOptIn(const CTxMemPoolEntry &entry, CTxMemPool &pool);
+
+#endif // BITCOIN_POLICY_RBF_H