Would it be reasonable to say that all non-directory refs will come from the same backend and extend the API so ```lookup``` takes a list of URIs? I've been running into some performance issues lately with MPD commands timing out on my client; when I did some (rough) profiling it seemed the queues between threads were the bottleneck. That in itself is a whole can of worms, and maybe a different PR, but it's kind of relevant here when there's potential for a ton of lookups going back and forth.