R1:4d0935ba5e69

Rate limit requests by IP

Summary:
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…