Use `Clock`-Interface (PSR-20) in `PSR-6` `CacheItem`

[boesing]

Since we do work with times within the PSR-6 CacheItem, we could consider adding the possibility to inject a Clock-Interface to the CacheItemPoolDecorator.

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Not important, but we should consider using a clock in future 🤔

Originally posted by @Ocramius in #199 (comment)