Fixes T3923. On secure.phabricator.com, we occasionally get slowed to a crawl when someone runs a security scanner against us, or 5 search bots decide to simultaneously index every line of every file in Diffusion.
Every time a user makes a request, give their IP address some points. If they get too many points in 5 minutes, start blocking their requests automatically for a while.
We give fewer points for logged in requests. We could futher refine this (more points for a 404, more points for a really slow page, etc.) but let's start simply.
Also, provide a mechanism for configuring this, and configuring the LB environment stuff at the same time (this comes up rarely, but we don't have a good answer right now).
