Refactor all callers (often replacing with more appropriate fn)
Also, added for_branch_setting method to old mongo and removed some
branch verification tests
Conflicts:
common/lib/xmodule/xmodule/modulestore/mongo/draft.py
common/lib/xmodule/xmodule/modulestore/split_mongo/split_draft.py
common/lib/xmodule/xmodule/modulestore/xml_importer.py