The lack of _for_write = True assignment in bulk_update prior to
accessing self.db resulted in the db_for_read database being used to
wrap batched UPDATEs in a transaction.
Also tweaked the batch queryset creation to also ensure they are
executed against the same database as the opened transaction under all
circumstances.
Refs #23646, #33501.
Thanks to Adam Johnson, Carlton Gibson, Mariusz Felisiak, and Raphael
Michel for mentoring this Google Summer of Code 2019 project and
everyone else who helped with the patch.
Special thanks to Mads Jensen, Nick Pope, and Simon Charette for
extensive reviews.
Co-authored-by: Mariusz Felisiak <felisiak.mariusz@gmail.com>