hasattr(request, '_cached_user') check no longer works

[Collin Anderson]

Before ​https://github.com/django/django/commit/2f01079, it was possible to check to see whether the lazy user has been evaluated or not using hasattr(request, '_cached_user'). This was undocumented, but used a lot: ​https://github.com/search?q=hasattr%28request%2C+%27_cached_user%27%29&type=Code

I think we should try to maintain backwards compatibility for that that check, and document and test it for the future. (Or if nothing else, document and test the new behavior and add a release note about the change.)

This is really helpful in middleware to be able to check this, so you can access the user if it has already been accessed, but can avoid fetching user if it hasn't already been fetched.