Refactor split migrator

LMS-2936

Removed a bunch of loc_mapper code
Cleaned up offering now being course and run in a few places
Removed extra db queries

Conflicts:
	common/lib/xmodule/xmodule/modulestore/tests/test_publish.py
Excellent idea. I think you're right that I don't need source_modulestore at least. (Too literally converting existing code). The default_store option would improve it but I'm blocked on that until merges occur.