With APCu 5+, use `apcu_*` function to examine cache state
Summary: Ref T9640. APCu 5.0+ (for PHP7) uses `apcu_*` functions instead of `apc_` functions. Test for function existence and call the appropriate functions.
Test Plan: {F2352695}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T9640
Differential Revision: https://secure.phabricator.com/D17198