Problem/Motivation
#3442009: OOP hooks using attributes and event dispatcher is in, and it's possible now to move hooks to services. But only in modules at the moment.
To make system module smaller, specifically and possibly also further on make more things components it would be very valuable to make components more self-contained.
#3383487: Add CronSubscriberInterface so that services can execute cron tasks directly and specifically system_cron() is a good example. It calls out to several components like caching, flood, key value. While the #Hook attribute is still a dependency on Drupal or at least another component, it would allow us to move code together.