Update download controller to use solr

Moving this pull request to projecthydra/sufia from https://github.com/psu-stewardship/sufia/pull/10

We recently implemented a similar downloads controller that we used for thumbnails (similar to you folks), and found that we got the same functionality at a significantly quicker pace (2-4x) if we hit up solr rather than ActiveFedora::Find. Thought I'd share. Seems to pass all your tests, but you'd know best if this'd affect you in any undue way.

Note: This includes a commit from mjgiarlo from the branch I actually forked this from - wasn't immediately clear to me how to remove that without deleting my previous fork, reforking, and redoing the commit.