Make PING skip the flood queue just like PONG does

It's unlikely server has nothing to send at all for all the other messages ZNC is sending and that there's nothing happening server-side too. But PING at least is guaranteed to give the response, so ZNC shouldn't disconnect too eagerly

src/IRCNetwork.cpp

@@ -51,7 +51,7 @@ class CIRCNetworkPingTimer : public CCron {
 
         if (pIRCSock &&
             pIRCSock->GetTimeSinceLastDataTransaction() >= uFrequency) {
-            pIRCSock->PutIRC("PING :ZNC");
+            pIRCSock->PutIRCQuick("PING :ZNC");
         }
 
         const vector<CClient*>& vClients = m_pNetwork->GetClients();