Use request cache for overriding branch settings LMS-11125

https://openedx.atlassian.net/browse/LMS-11125

@cpennington Please review.
Theoretically, a thread-local would be sufficient for the `branch_setting` contextmanager.  However, rather than creating a new thread-local variable, I have the modulestores make use of the `request_cache` thread-local that has already been created and passed down.

Updating the Split Modulestore to use the new branch_setting implementation will be a separate PR.
Now that you've pulled this out into a mixin, I think it makes sense to either get rid of the `course_id` parameter, or use it in the implementation.