Optimize the batch_write operation

[naschorr]

Writing the DynamoDB specified 25 items per batch is super slow, however the docs mention that using multiple threads for the batch operation is totally fine.

Alternatively, the native batch_writer seems to be able to handle many more than 25 items at once. More experimentation is needed.

[naschorr]

As a follow up, it looks like even if you pass batch_writer N > 25 items, it still processes it in chunks of 25. That said (anecdotally), I did notice that data copying seemed to be happening slightly quicker. Maybe a couple minutes faster per chunk of ~7000 items from the AWS instance.

That said, I didn't robustly profile the operation, so it could just be a fluke.