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.
`import_only` is for XML import, as opposed to an author uploading a new asset. The difference is that the `edited_{on,by,by_mail}` information would not be updated on an XML import - the imported information would be used instead. That's *all* the flag does. I'm open to better names...