Add tests for OnCommit batch operations

[scale-tone]

Check combination of all operations for different records
Check what happens when you try to add same record multiple times (should throw)
Check what happens when you add + modify within 1 "transaction" (should save as modified, check logs that only 1 dynamoDb operation is performed)
Check what happens when you add + delete within 1 "transaction" (should be deleted, check logs that only 1 dynamoDb operation is performed)
Check what happens when you add 40 records + modify 40 records + delete 40 records (DynamoDb supports only batches of 30 operations and less), should auto-batch