(websocket) add a positive number to the heartbeat message sent, incremented each time the heartbeat is sent

ixwebsocket/IXWebSocketTransport.h

@@ -212,6 +212,7 @@ namespace ix
 
         static const int kDefaultPingIntervalSecs;
         static const std::string kPingMessage;
+        std::atomic<uint64_t> _pingCount;
 
         // We record when ping are being sent so that we can know when to send the next one
         mutable std::mutex _lastSendPingTimePointMutex;