This adds cron code which looks for question previews that have not been
touched for more than 24 hours, and deletes them.
We try to delete previews immediately. For example if the user clicks
start again, then we immediately delete their previous preview. However,
we can't do that if they just close the preview window. Hence we need
some cron code to clean up old preview that have got left lying around.
Normally, this code will not have much to do, so it will be very fast,
so we can afford to run it every cron.
This has been implemented in such a way that in future it will be easy
to add other cron code to the question bank.
Sadly, to make this work on MySQL, we require a horrible hack in the
already hacky delete_usage_records_for_mysql function.
1. We need this so that, for example, when previewing a question from
the quiz editing page, the preview uses the filter settings, theme and
language set for that quiz and course.
Tim Hunt