Bug fixes for BatchGetItem UnprocessedKeys and BatchWriteItem v2 Deletes.

This revision addresses these two Issues:

AlternatorDBHandler.batchWriteItemV2() - fails for delete items
https://github.com/mboudreau/Alternator/issues/69

BatchGetItems - UnprocessedKeys is Null
https://github.com/mboudreau/Alternator/issues/42