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 :-)
Can we have the situation where the nr_requests is = 0, but still be here?
Seems like we should skip the printing all-together if no requests have been server.
Otherwise we are going to be dividing upstream_total_latency_ms (which I assume to be 0) for 1, skewing the average down?