Bugfix: remove conflicting transactions from memory pool

When a transaction A is in the memory pool, while a transaction B
(which shares an input with A) gets accepted into a block, A was
kept forever in the memory pool.

This commit adds a CTxMemPool::removeConflicts method, which
removes transactions that conflict with a given transaction, and
all their children.

This results in less transactions in the memory pool, and faster
construction of new blocks.

src/main.h

@@ -1814,7 +1814,8 @@ public:
 
     bool accept(CTransaction &tx, bool fCheckInputs, bool* pfMissingInputs);
     bool addUnchecked(const uint256& hash, CTransaction &tx);
-    bool remove(CTransaction &tx);
+    bool remove(const CTransaction &tx, bool fRecursive = false);
+    bool removeConflicts(const CTransaction &tx);
     void clear();
     void queryHashes(std::vector<uint256>& vtxid);
     void pruneSpent(const uint256& hash, CCoins &coins);