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.
Yeah, @cpennington, I was confused too b/c I thought the middleware did it via a contextmanager like wrapper. Perhaps we should add a story in the clean up to move the hostname branch setting override to middleware?