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:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9640

Differential Revision: https://secure.phabricator.com/D17198