Limit setAskFor and retire requested entries only when a getdata returns.

The setAskFor duplicate elimination was too eager and removed entries
 when we still had no getdata response, allowing the peer to keep
 INVing and not responding.

src/net.h

@@ -59,6 +59,8 @@ static const bool DEFAULT_UPNP = false;
 #endif
 /** The maximum number of entries in mapAskFor */
 static const size_t MAPASKFOR_MAX_SZ = MAX_INV_SZ;
+/** The maximum number of entries in setAskFor (larger due to getdata latency)*/
+static const size_t SETASKFOR_MAX_SZ = 2 * MAX_INV_SZ;
 
 unsigned int ReceiveFloodSize();
 unsigned int SendBufferSize();