Migrate courses from old mongo to split

In addition to the migration,
* fixed lots of minor bugs and moved some things to new kvs and fields patterns
* began the idea of a with_new_version ability to not spawn a version for every minor change
* had to deal with lots of cache issues from not spawning new versions (which also found some bugs)
* added ability for new get_item to take old Location() and use mapper to translate (inject mapper during config)

@cahrens @cdodge @cpennington pls review, delegate, or defer :-)