R1:3f53718d107f

Modularize rate/connection limits in Phabricator

Summary:
Depends on D18702. Ref T13008. This replaces the old hard-coded single rate limit with multiple flexible limits, and defines two types of limits:

- Rate: reject requests if a client has completed too many requests recently.
- Connection: reject requests if a client has too many more connections than disconnections recently.

The connection limit adds +1 to the score for each connection, then adds -1 for each disconnection. So the overall…