Print debug message when a client is connected

src/app/handlers/zmq_websocket_handler.py

@@ -13,6 +13,8 @@ class ZMQWebSocketHandler(WebSocketHandler):
         super().__init__(application, request, **kwargs)
 
     def open(self, *args, **kwargs):
+        logging.debug('WebSocket connection initiated')
+
         def zmq_callback(msg_parts):
             anchor, data = msg_parts
             logging.debug('Received on pull socket: {}'.format(data.decode()))