R1:7ccc4cea43e3

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