rename _blocking to _backgroundThreadRunning and invert the naming

ixwebsocket/IXWebSocket.h

@@ -154,7 +154,7 @@ namespace ix
         static OnTrafficTrackerCallback _onTrafficTrackerCallback;
 
         std::atomic<bool> _stop;
-        std::atomic<bool> _blocking;
+        std::atomic<bool> _backgroundThreadRunning;
         std::atomic<bool> _automaticReconnection;
         std::thread _thread;
         std::mutex _writeMutex;