R1:3698e4a14f6c

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…