Handle max read and write batch sizes

[vkartaviy]

It would be great to handle max read and write batches size limitations as https://github.com/awslabs/dynamodb-data-mapper-js do that...

When you are trying to get more than 100 items it put the rest into the pending list which is processed together with unprocessed keys automatically.

[fredspivock]

This would be awesome, reduce the complexity for end users.