Speed up model rendering with apps.ready = False

[Marten Kenbeek]

Whenever a model is rendered, the cache of several values in model._meta is cleared on each known model. This has a severe performance impact when rendering large amounts of models.

When apps.ready is False, the cache of model meta options is not cleared. This speeds up the initial rendering of the test suite significantly. The cache can then be cleared manually when all models are rendered, and apps.ready can be set back toTrue.

Results timing django.test.runner.DiscoverRunner.setup_databases:

apps.ready == True  - 42 seconds
apps.ready == False - 16 seconds

[Markus Holtermann <info@…>]

In 888c9b6429a44824078a49fb1ebacf2e950cd887:

Fixed #24397 -- Sped up rendering multiple model states.

Set apps.ready to False when rendering multiple models. This prevents
that the cache on Model._meta is expired on all models after each time a
single model is rendered. Prevented that Apps.clear_cache() refills the
cache on Apps.get_models(), so that the wrong value cannot be cached
when cloning a StateApps.

[Marten Kenbeek]

Additional PR: ​https://github.com/django/django/pull/4458

[Markus Holtermann]

Reopen due to further improvements.

[Tim Graham]

Left some mostly cosmetic comments. Please advance to "Ready for Checkin" after updating.

[Marten Kenbeek]

I'm not too sure what the error message should be changed to, so I'll leave it as is.

[Tim Graham <timograham@…>]

In 039d7881:

Refs #24397 -- Sped up model reloading in ProjectState.

Created bulk_update() context manager on StateApps. Sped up unregistering
models in reload_models() by using this context mananger.