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.h

@@ -17,6 +17,7 @@
 #include "sync.h"
 #include "uint256.h"
 
+#include <atomic>
 #include <deque>
 #include <stdint.h>
 
@@ -413,6 +414,8 @@ public:
     // Used for BIP35 mempool sending, also protected by cs_inventory
     bool fSendMempool;
 
+    // Last time a "MEMPOOL" request was serviced.
+    std::atomic<int64_t> timeLastMempoolReq;
     // Ping time measurement:
     // The pong reply we're expecting, or 0 if no pong expected.
     uint64_t nPingNonceSent;