Invoked the application callable method and start
        the ``arbiter`` if it wasn't already started.

        It returns a :class:`~asyncio.Future` called back once the
        application/applications are running. It returns ``None`` if
        called more than once.

def start(self, exit=True):
        """Invoked the application callable method and start
        the ``arbiter`` if it wasn't already started.

        It returns a :class:`~asyncio.Future` called back once the
        application/applications are running. It returns ``None`` if
        called more than once.
        """
        on_start = self()
        actor = arbiter()
        if actor and on_start:
            actor.start(exit=exit)
            if actor.exit_code is not None:
                return actor.exit_code
        return on_start