Scalatra cache

Re-opening with updated code. Many thanks to @rossabaker and @casualjim for holding my hand thru this!
@seratch I agree with your point, but Guava doesn't seem to do per-key TTL. I can't find a common thread to handle TTL cleanly across multiple back-ends, so for now I nuked TTL entirely. I'll revisit it in my next PR, it's easy enough to plug another backend for now and handle TTLs there.