Generic action for cron events

[batmoo]

It would be nice to have a generic action in wp-cron.php to allow plugins to hook into cron processing without knowing names of specific registered cron actions, e.g. a plugin that logs all executed cron events.

Patch adds a new action (did_cron), which is executed after the actual cron event and passes in the hook, timestamp, and arguments for the cron event.

Duplicate of #18471.