Update rate limiting for APCu and X-Forwarded-For

Summary:
Ref T12612. This updates the rate limiting code to:

  - Support a customizable token, like the client's X-Forwarded-For address, rather than always using `REMOTE_ADDR`.
  - Support APCu.
  - Report a little more rate limiting information.
  - Not reference nonexistent documentation (removed in D16403).

I'm planning to put this into production on `secure` for now and then we can deploy it more broadly if things work well.

Test Plan:
 - Enabled it locally, used `ab -n 100` to hit the limit, saw the limit enforced.
 - Waited a while, was allowed to browse again.

Reviewers: chad, amckinley

Reviewed By: amckinley

Maniphest Tasks: T12612

Differential Revision: https://secure.phabricator.com/D17758

src/docs/user/configuration/configuring_preamble.diviner

@@ -1,8 +1,7 @@
 @title Configuring a Preamble Script
 @group config
 
-Adjust environmental settings (SSL, remote IP, rate limiting) using a preamble
-script.
+Adjust environmental settings (SSL, remote IPs) using a preamble script.
 
 Overview
 ========