Support APCu with apcu_* functions

APC and APCu is supported according to https://docs.civicrm.org/sysadmin/en/latest/setup/cache/#config-ref but it currently only supports apc_* functions which are backwards-compatible with not compatible with APC. But some setups may only have the newer apcu_* functions so the class should try both functions. APCu only has apcu_* functions, but still uses apc. for the configuration.

UPDATE: discovered there are some differences between the two in how they handle things, which isn't surprising. So depending on how much they differ, may require different classes.