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 :-)
It's a valid question, we never reach this point in the code when nr_requests is 0 actually, or we shouldn't ... if the logic here: https://github.com/plain-vanilla-games/ngx_http_accounting_module/pull/5/files#diff-f07626ecb062b1ba8f62cde9857c5feaR163 is correct.

Since we shouldn't get here I assume it *should* be safe to just divide blindly, but I'm reluctant to do it because if the logic above is broken it might produce a crash :-/