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
this comment is odd. When is "eventually"? Can you say, "when X, Y, Z is complete" instead?

You then tack on, "from there they default to None" - this seems like an odd unexplained parenthetical, can you add a why here?