Issue #2999614: Use Drupal core time service with fallback

Closes #2999614

it should inject the time service as each get() (validation) makes a call to singleton and each invalidation doing it

so static call instead of variable get (with fallback to syscall) is a viable overhead of every cache item and tag

We cannot inject the time service since it isn't guaranteed to be always available (see fabianx comment at https://drupal.org/i/2999614#comment-12787538), but I can add a static cache, indeed.

I also didn't see any reason to be public here, but now it is already part of the API, we shouldn't change the visibility anymore.

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• 5b885df7 - Issue #2999614: Add static cache for request timestamp.

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• 20f8103b - Issue #2999614: Wrong variable scope

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