text: Title: How to control /socket.io/ endpoint
  Body: Hello,
  
  This is less of an issue and more of a question, but is it possible to control the `/socket.io/` endpoint?
  
  I am using `Flask-SocketIO` for authenticated users only, using the `authenticated_only` function wrapper:
  ```def authenticated_only(f):
      @functools.wraps(f)
      def wrapped(*args, **kwargs):
          if not current_user.is_authenticated:
              disconnect()
          else:
              return f(*args, **kwargs)
      return wrapped
  ```
  
  All `@socketio.on` handlers are making use of this wrapper, including `@socketio.on('connect')`.
  
  If an unauthenticated user makes a direct `GET` request to my server to the `/socket.io/` endpoint, for example `/socket.io/?EIO=3&transport=polling&t=MqK2tJx` this request will time-out and eventually result in a 502 error (`socket.io` is behind `nginx`).  But it seems to still be "stuck" in the async queue.
  
  Is it possible to control the `/socket.io/` and immediately return a `403` for unauthenticated users?
  
  Otherwise I eventually end up with errors as a core is "stuck" trying to manage this request `[DANGER] async queue is full !!!`