Transaction hash caching

Use CBlock's vMerkleTree to cache transaction hashes, and pass them
along as argument in more function calls. During initial block download,
this results in every transaction's hash to be only computed once.

src/rpcrawtransaction.cpp

@@ -517,7 +517,7 @@ Value sendrawtransaction(const Array& params, bool fHelp)
         // Not in block, but already in the memory pool; will drop
         // through to re-relay it.
     } else {
-        SyncWithWallets(tx, NULL, true);
+        SyncWithWallets(hashTx, tx, NULL, true);
     }
     RelayMessage(CInv(MSG_TX, hashTx), tx);