Handle mempool requests in send loop, subject to trickle

By eliminating queued entries from the mempool response and responding only at
trickle time, this makes the mempool no longer leak transaction arrival order
information (as the mempool itself is also sorted)-- at least no more than
relay itself leaks it.

src/net.h

@@ -406,6 +406,8 @@ public:
     // Used for headers announcements - unfiltered blocks to relay
     // Also protected by cs_inventory
     std::vector<uint256> vBlockHashesToAnnounce;
+    // Used for BIP35 mempool sending, also protected by cs_inventory
+    bool fSendMempool;
 
     // Ping time measurement:
     // The pong reply we're expecting, or 0 if no pong expected.