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.cpp

@@ -2370,6 +2370,7 @@ CNode::CNode(SOCKET hSocketIn, const CAddress& addrIn, const std::string& addrNa
     hashContinue = uint256();
     nStartingHeight = -1;
     filterInventoryKnown.reset();
+    fSendMempool = false;
     fGetAddr = false;
     nNextLocalAddrSend = 0;
     nNextAddrSend = 0;