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
I couldn't grok why I'd deliberately left them off originally. I was going to get rid of the waiting adoption stuff and use this logic to populate the children, but I couldn't quite work that out.