Allow the InRequest key-value cache to have a simple key limit

Summary: Performance of the new graph cache is okay, but pulling data from APC is more expensive than I expected. I'm going to try to tune that by doing some in-request caching, since all requests normally examine the same bucket at first.

Test Plan: Added and executed unit tests.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9079