(socket server) add a callback to the ConnectionState to be invoked when the connection is terminated. This will be used by the SocketServer in the future to know on time that the associated connection thread can be terminated.

ixwebsocket/IXSocketServer.h

@@ -85,6 +85,7 @@ namespace ix
         // background thread to wait for incoming connections
         std::thread _thread;
         void run();
+        void onSetTerminatedCallback();
 
         // background thread to cleanup (join) terminated threads
         std::atomic<bool> _stopGc;