def subscribe_async(self, subject, **kwargs):
        """
        Schedules callback from subscription to be processed asynchronously
        in the next iteration of the loop.
        """
        kwargs["is_async"] = True
        sid = yield self.subscribe(subject, **kwargs)
        raise tornado.gen.Return(sid)
Schedules callback from subscription to be processed asynchronously
        in the next iteration of the loop.