Do not use mempool for GETDATA for tx accepted after the last mempool req.

The ability to GETDATA a transaction which has not (yet) been relayed
 is a privacy loss vector.

The use of the mempool for this was added as part of the mempool p2p
 message and is only needed to fetch transactions returned by it.

src/net.cpp

@@ -2396,6 +2396,7 @@ CNode::CNode(SOCKET hSocketIn, const CAddress& addrIn, const std::string& addrNa
     fRelayTxes = false;
     fSentAddr = false;
     pfilter = new CBloomFilter();
+    timeLastMempoolReq = 0;
     nPingNonceSent = 0;
     nPingUsecStart = 0;
     nPingUsecTime = 0;