Put an in-request cache in front of APC

Summary: Most requests examine the same buckets, especially the first bucket. Let them just read it out of request cache.

Test Plan: Observed most bucket fetches resolving in <10us instead of <10ms.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9080