Use request cache for overriding branch settings.

@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.
any idea why pylint complains that `TestMixedModuleStore.test_branch_setting: Instance of 'TestMixedModuleStore' has no 'problem_x1a_1' member` ?