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

@@ -2142,6 +2142,7 @@ CNode::CNode(SOCKET hSocketIn, CAddress addrIn, std::string addrNameIn, bool fIn
     nStartingHeight = -1;
     fGetAddr = false;
     fRelayTxes = false;
+    fSentAddr = false;
     pfilter = new CBloomFilter();
     nPingNonceSent = 0;
     nPingUsecStart = 0;