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 number is how many open connections they have, at least approximately.

Supporting multiple limits will let us do limiting by Hostname and by remote address (e.g., a specific IP can't exceed a low limit, and all requests to a hostname can't exceed a higher limit).

Configuring the new limits looks something like this:

PhabricatorStartup::addRateLimit(new PhabricatorClientRateLimit())
  ->setLimitKey('rate')
  ->setClientKey($_SERVER['REMOTE_ADDR'])
  ->setLimit(5);

PhabricatorStartup::addRateLimit(new PhabricatorClientConnectionLimit())
  ->setLimitKey('conn')
  ->setClientKey($_SERVER['REMOTE_ADDR'])
  ->setLimit(2);

Test Plan:

• Configured limits as above.
• Made a lot of requests, got cut off by the rate limit.
• Used curl --limit-rate -F 'data=@the_letter_m.txt' ... to upload files really slowly. Got cut off by the connection limit. With enable_post_data_reading off, this correctly killed the connections before the uploads finished.
• I'll send this stuff to secure before production to give it more of a chance.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13008

Differential Revision: https://secure.phabricator.com/D18703