Use request cache for overriding branch settings.

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.
@cpennington Yes, we can make the branch setting override be course specific.  Hence, the presence of the `course_id` parameter.  However, I think it's something we can add in the future, rather than making this story even bigger.