basic implementation of transaction replacement. requires cleanup and testing

src/replacementpool.h

@@ -0,0 +1,68 @@
+// Copyright (c) 2018 The Komodo Developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef KOMODO_REPLACEMENTCACHE_H
+#define KOMODO_REPLACEMENTCACHE_H
+
+#include "coins.h"
+#include "primitives/transaction.h"
+
+
+enum CTxReplacementPoolResult { RP_NotReplace, RP_Valid, RP_Invalid };
+
+class CTxReplacementPoolItem
+{
+public:
+    CTransaction tx;
+    int startBlock;
+    uint64_t priority;
+    uint32_t replacementWindow;
+
+    CTxReplacementPoolItem() {}
+
+    CTxReplacementPoolItem(const CTransaction &_tx, int _startBlock) {
+        tx = _tx;
+        startBlock = _startBlock;
+        priority = 0;
+        replacementWindow = 0;
+    }
+
+    int GetTargetBlock()
+    {
+        return startBlock + replacementWindow;
+    }
+};
+
+/**
+ * CTxReplacementPool stores valid-according-to-the-current-best-chain (??? do we need to do this?)
+ * transactions that are valid but held for period of time during which
+ * they may be replaced.
+ *
+ * Transactions are added when they are found to be valid but not added
+ * to the mempool until a timeout.
+ *
+ * Replacement pool is like another mempool before the main mempool.
+ */
+class CTxReplacementPool
+{
+private:
+    // A potential replacement is first stored here, not in the replaceMap.
+    // This is in case some other checks fail, during AcceptToMemoryPool.
+    // Later on, if all checks pass, processReplacement() is called.
+    
+    /* Index of spends that may be replaced */
+    std::map<COutPoint, CTxReplacementPoolItem> replaceMap;
+public:
+    CTxReplacementPoolResult replace(CTxReplacementPoolItem &item);
+
+    // Remove and return all transactions up to a given block height.
+    void removePending(int height, std::vector<CTransaction> &txs);
+
+    bool lookup(uint256 txHash, CTransaction &tx);
+};
+
+
+extern CTxReplacementPool replacementPool;
+
+#endif // KOMODO_REPLACEMENTCACHE_H