Harvest request stats from upstream Nginx log file.

Still experimental, but this gives us *much* more accurate real-time stats on
file requests and referrers than we had previously, and paves the way for
intelligent auto-throttling and auto-blacklisting.

If an upstream log file is unavailable or can't be read, requests will still be
tracked internally.
Much more detailed request stats.

Inching closer to autothrottling.