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
Every xblock's id and location has a version guid in split. Split uses it to compute version conflicts. (which raises the question why I'm blanking it out here.)