Phase 1 of adding asset metadata saving to old Mongo

@dmitchell @cpennington Here's the first phase of my work. Look at the tests to see what is implemented and what isn't yet.
It's currently not used anywhere except tests. Perhaps the key problem here is that I'm implementing bits before they're actually used. So I'm not positive if I need to examine the content or not - it depends on what's needed down the line.

How about this?: It's at least obvious how it works now (just checking asset_id equivalence). Maybe I leave it for now and change it if/when I need to change it later?