Refactor Drupal\Core\Cron

Problem/Motivation

Pretty simple idea. The Cron class could do with a bit of a tidy, everything just lives in the run() method. As well as just having a general spring clean, splitting some of the logic out to separate methods will allow better DX for people that want to swap this class out for their own class, extending the Drupal\Core\Cron class.

Proposed resolution

Separate some logic out into separate methods, maybe something like the addition of:

- setCronLastTime
- processQueues
- invokeCronHandlers

Remaining tasks

Reviews, consensus.

User interface changes

None

API changes

None