MixedModulestore wraps most getters, update_item, delete_item
with code to translate between addressing schemes based on app
and persistence layer addressing scheme specification.

STUD-1206
Added test for defaulting use_locations
update_item and other refactorings continued
Review-driven changes (to-be-squashed)
Merge pull request #2356 from edx/dhm/mixed_ms_wrapper

MixedModulestore wraps most getters, update_item, delete_item