Previously, quiz statistics processing happened on a scheduled task.
This task looked for all quizzes with completed attempts, then
determined if those quizzes had a statistics calculation that's newer
than the most recent attempt, then ran the statistics calculation if
needed. It was hard coded to stop processing after 1 hour.
The queries involved in determining which quizzes needed processing
weren't terribly efficient, and combined with the 1 hour limit this made
the statistics unusable on large sites, where they are the most useful.
This converts the scheduled task to an ad-hoc task, and uses an event
observer for mod_quiz\event\attempt_submitted to queue a task when
it is needed. This removes the need for a query to work out what needs
processing, and allows the task processing to be scaled up as needed.