APCu compatibility (maybe v2)

The problem being that the plugin still uses the function names for APC – like apc_fetch – while the package was discontinued in 2013 in favour of APCu.

For the latest deployments (like Ubuntu 16.04) the compatibility setting is dropped by default.

There are multiple solutions to match this: either support both apcu_fetch and apc_fetch or only the former or write some wrapper for caching.
The way it is now is not future-proof and not compatible with recent APC deployments.

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