save lock state in contentstore

Does not handle export/import as that's a much bigger change and I wanted to get this in to unblock other work. The crux of the change is in mongo.py. There are unit tests for get/set of attr, but there are no unit tests for lock set during asset save and retrieved during asset find yet. I'd like to know whether to add these to the xmodule test file or put them in the cms/django ones.

@cdodge @cahrens pls review