def _really_start_hb(self):
        """callback for delayed heartbeat start
        
        Only start the hb loop if we haven't been closed during the wait.
        """
        if self._beating and not self.hb_stream.closed():
            self._hb_periodic_callback.start()
callback for delayed heartbeat start
        
        Only start the hb loop if we haven't been closed during the wait.