Avoid re-downloading media that hasn't changed

[kueda]

I'm pretty sure the update process downloads media for all observations that have been updated, even if that media has not been updated. I suspect we could be smarter about choosing media to update by looking at the updated_at field of the photo and sound records and not re-download media that hasn't been updated since the updated_since value we used.

[kueda]

Oops, forgot that the iNat API doesn't show updated_at for photos, so that would need to change first.