Tracking upstream latency separately

This PR attempts to retrieve the upstream latency from request->upstream_states in a very naive and potentially dangerous way.  Pushing this here due to our slightly stupid build process :-)
Ah, okay. I see the check above. We'll be returning before if the stats->nr_requests is not > 0.
I understand the reluctance, but I think it could be pretty safely removed - we are basically re-checking the exact same thing. :-)