Only send one GetAddr response per connection.

This conserves resources from abusive peers that just send
 getaddr in a loop. Also makes correlating addr messages
 against INVs less effective.

src/net.cpp

@@ -2384,6 +2384,7 @@ CNode::CNode(SOCKET hSocketIn, const CAddress& addrIn, const std::string& addrNa
     nNextAddrSend = 0;
     nNextInvSend = 0;
     fRelayTxes = false;
+    fSentAddr = false;
     pfilter = new CBloomFilter();
     nPingNonceSent = 0;
     nPingUsecStart = 0;