fix(django-cf): stop using DJANGO_ALLOW_ASYNC_UNSAFE from django-cf backends

[ryanking13]

Stop using discouraged DJANGO_ALLOW_ASYNC_UNSAFE flag and mark django-cf backends async-safe.

Since django by default wraps all the database methods with @async_unsafe decorator, this includes a slightly hacky solution that unwraps decorators of those methods.

[ask-bonk]

LGTM

github run

[dom96]

Hmm, isn't this effectively the same as setting DJANGO_ALLOW_ASYNC_UNSAFE?

[ryanking13]

Hmm, isn't this effectively the same as setting DJANGO_ALLOW_ASYNC_UNSAFE?

Partially, but a few things are different.

1. There can be other async-unsafe places, e.g. django middlewares or even another database backend (if they are using multiple databases.... though it should be very rare). This change limits the impact only to the django-cf backends to prevent unexpected side effects.

2. Since the logic it backed in to the database backend itself, users do not need to set the env variable manually. Previously, users need to set it manually or had to inherit DjangoCF class which I don't think is very optimal.