Rename `PhutilKeyValueCache` subclasses

Summary: Ref T5655. Rename `PhutilKeyValueCache` subclasses for consistency. I intentionally left `PhutilKeyValueCacheNamespace`, `PhutilKeyValueCacheProfiler`, `PhutilKeyValueCacheProxy` and `PhutilKeyValueCacheStack` because these seem to be special cases.

Test Plan: `arc lint` and `arc unit`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D10155