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.cpp

@@ -0,0 +1,46 @@
+// 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.
+
+#include "policy/rbf.h"
+
+bool SignalsOptInRBF(const CTransaction &tx)
+{
+    BOOST_FOREACH(const CTxIn &txin, tx.vin) {
+        if (txin.nSequence < std::numeric_limits<unsigned int>::max()-1) {
+            return true;
+        }
+    }
+    return false;
+}
+
+bool IsRBFOptIn(const CTxMemPoolEntry &entry, CTxMemPool &pool)
+{
+    AssertLockHeld(pool.cs);
+
+    CTxMemPool::setEntries setAncestors;
+
+    // First check the transaction itself.
+    if (SignalsOptInRBF(entry.GetTx())) {
+        return true;
+    }
+
+    // If this transaction is not in our mempool, then we can't be sure
+    // we will know about all its inputs.
+    if (!pool.exists(entry.GetTx().GetHash())) {
+        throw std::runtime_error("Cannot determine RBF opt-in signal for non-mempool transaction\n");
+    }
+
+    // If all the inputs have nSequence >= maxint-1, it still might be
+    // signaled for RBF if any unconfirmed parents have signaled.
+    uint64_t noLimit = std::numeric_limits<uint64_t>::max();
+    std::string dummy;
+    pool.CalculateMemPoolAncestors(entry, setAncestors, noLimit, noLimit, noLimit, noLimit, dummy, false);
+
+    BOOST_FOREACH(CTxMemPool::txiter it, setAncestors) {
+        if (SignalsOptInRBF(it->GetTx())) {
+            return true;
+        }
+    }
+    return false;
+}