Support bulk insert

[surister]

Django supports notifying the driver that it wants to create objects in bulk with bulk_create, e.g:

>>> objs = Entry.objects.bulk_create(
...     [
...         Entry(headline="This is a test"),
...         Entry(headline="This is only a test"),
...     ]
... )

In CrateDB bulk inserts in the http protocol can be implemented with bulk_args.

We should bridge both PRs to implement fast bulk insertion