batch_import_iter() should call a progress callback

[Jason Gerard DeRose]

Currently if you're importing large files (say 4GB), the progress bars go too long without any feedback as we only update the progress bars when the next file is complete. Many people have raised this as their first (and sometimes only) complaint.

I've put this off because it requires us to cross the streams between filestore and dmedia in a way I'm generally trying to avoid, but sucky UX is sucky UX, so it needs to be fixed.

Thanks @WindUpToy for bringing it to my attention again... I think this time it stuck :^)