Feature: Batch Delete

[ricky-sb]

We have a batch_put operation, but no batch_delete operation. The architecture here might be a bit difficult to integrate.

Here's how I'm currently doing it. In my main class:

items_to_delete = MyModel.query(client_id='test123')
with MyModel.Table.table.batch_writer() as writer:
    for item in items_to_delete:
        delete_item_kwargs = {}
        item._add_hash_key_values(delete_item_kwargs)
        item._normalize_keys_in_kwargs(delete_item_kwargs)
        writer.delete_item(Key=delete_item_kwargs)

I would like to do something like this:

items_to_delete = list(MyModel.query(client_id='test123'))
MyModel.batch_delete(items_to_delete)

and it would essentially be syntatic sugar for that.