Updated the download controller to use load_instance_from_solr for speed

Fixed pull request from https://github.com/projecthydra/sufia/pull/83 now based off the master of this repo.

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.