Service the observable's inBox and outBox

        :return: the number of messages successfully serviced

async def service_observable(self, limit) -> int:
        """
        Service the observable's inBox and outBox

        :return: the number of messages successfully serviced
        """
        if not self.isReady():
            return 0
        o = self._service_observable_out_box(limit)
        i = await self._observable.serviceQueues(limit)
        return o + i