Fixed #29613 -- Fixed --keepdb on PostgreSQL if the database exists and the user can't create databases.

Regression in e776dd2d.

Thanks Tim Graham for the review.

Regression in 7d96f0c4.

A race condition happened when the object didn't already exist and
another process/thread created the object before update_or_create()
did and then attempted to update the object, also before update_or_create()
saved the object. The update by the other process/thread could be lost.

Thanks Akshesh Doshi for the initial implementation.

- Added mixin to make tests more DRY.
- Removed redundant equality tests.

Forwardport of cbf7e7dc from stable/2.1.x.

Forwardport of 734e2c65 from stable/2.1.x.

Regression in 3a148f95.

Refs #29600 -- Removed usage of django.utils.datetime_safe in migrations.

The only effect would be if items in Atom feeds had a published date
year of < 1000 (ensuring those years are padded with leading zeros).

Removed in 244cc401.