Add possibility to set a cacheKey in constructor and to reuse this key

[tsteur]

For example by developing a decorator.

This will bring a performance improvement as we do for each fetch, contains, save run a preg_match https://github.com/piwik/component-cache/blob/master/src/Lazy.php#L119 and a completeCacheKey which is expensive when executed a million of times (which we do) eg in BulkTracking or QueuedTracking. Also via Archiving etc.