Performance enhancements for django-admin commands to delete and clone course

NOTES: This is done to vastly speed up deleting and cloning courses which are django-admins I run often regarding courseware manipulations. The problem was that there was no metadata cache being setup so there was *alot* of DB thrashing happening. Also we need to add the course_id to a special list of courses which are known to have a lot of grouped write transactions on it, so that final metadata computation is done at the end (for clone_course in this case).

@cahrens @dmitchell can you review?