model caching: Delete cache rather than invalidate.

Otherwise, one can’t be sure that a method returns the correct value
other than to wait for one second. But this can’t be done in
production, where no method such that `wait_for_cache` exists.

Trello:
https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching
Cache: Applying the new model caching mechanism to the User model.

Trello:
https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching