Batch store offset per slice

[patriknw]

The TimestampOffset per slice can probably be stored in the background, or at least not in each saveOffset. It can be flushed on flush on size/time. After a restart we will see duplicates, which is fine, if storeSequenceNumbers is successful and storeTimestampOffsets fails or is performed later.

Also, we must group the batches, also the storeSequenceNumbers, into chunks that fit the DynamoDB batch limit.