Intializes the partition checkpoint and lease store and then calls run async.

async def start_async(self):
        """
        Intializes the partition checkpoint and lease store and then calls run async.
        """
        if self.run_task:
            raise Exception("A PartitionManager cannot be started multiple times.")

        partition_count = await self.initialize_stores_async()
        _logger.info("%r PartitionCount: %r", self.host.guid, partition_count)
        self.run_task = asyncio.ensure_future(self.run_async())