Setup check for apcu cache statistics

[juliusknorr]

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Is your feature request related to a problem? Please describe.

We noticed that with a too low apc.shm_size the local cache could be invalidated very frequently where apcu purges all keys. Frequent invalidation can potentially have an impact on performance so we should add a setup check to give hints to admins for that.

Describe the solution you'd like

• Warn about frequent invalidation mabye based on uptime and cache full count
• Other measurements depending on hit/miss rate, insert rate or cache growth if measurable could be also taken into account

Describe alternatives you've considered
Expose this to monitoring, but the serverinfo app already does that and likely gets overseen if admins don't dive too deep into metrics.

Additional context

Official script for getting insights https://raw.githubusercontent.com/krakjoe/apcu/master/apc.php