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 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).

Test Plan: Used `ab` and reloading over and over again to hit rate limits. Read documentation.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: chad, epriestley

Maniphest Tasks: T3923

Differential Revision: https://secure.phabricator.com/D8713