In PhutilAPCKeyValueCache, use APCu functions on APCu 5+

Summary: Ref T9640. Recent versions of APCu only define apcu_* functions.

Test Plan: See D17198, which has a screenshot of the cache full of data after this change.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9640

Differential Revision: https://secure.phabricator.com/D17199