Import/export modulestore-stored asset metadata as XML from/to exported course.

Export modulestore-stored asset metadata as XML to exported course.
Import asset metadata XML into course's modulestore.
Return empty values and log a warning instead of raising NotImplemented exceptions for XML modulestore asset metadata methods.
Optimize importing many items of asset metadata by avoiding multiple round-trips to MongoDB.
The return from `_find_course_assets()` is a dict with (at least) an `_id` key, which identifies the relevant Mongo doc if updates need to occur.
If there's also asset metadata stored, other keys will be the other asset types with values being lists of asset metadata.
I'll add this description to the docstring.