Gets the initial offset for processing the partition.

        :rtype: str

async def get_initial_offset_async(self): # throws InterruptedException, ExecutionException
        """
        Gets the initial offset for processing the partition.

        :rtype: str
        """
        _logger.info("Calling user-provided initial offset provider %r %r",
                     self.host.guid, self.partition_id)
        starting_checkpoint = await self.host.storage_manager.get_checkpoint_async(self.partition_id)
        if not starting_checkpoint:
            # No checkpoint was ever stored. Use the initialOffsetProvider instead
            # defaults to "-1"
            self.offset = self.host.eph_options.initial_offset_provider
            self.sequence_number = -1
        else:
            self.offset = starting_checkpoint.offset
            self.sequence_number = starting_checkpoint.sequence_number

        _logger.info("%r %r Initial offset/sequenceNumber provided %r/%r",
                     self.host.guid, self.partition_id, self.offset, self.sequence_number)
        return self.offset