This changes the cache purge code to select IDs for deletion
first, rather than using a subquery in a DELETE, since this
risks causing a deadlock in MySQL.
Previously, a set of calculated quiz statistics would only 'last' for
15 minutes. Then they would be considered invalid and not used.
Now, computed statistics are kept indefinitely. Instead, when a new
batch of values are computed for a particular set of settings, older numbers
for the same settings are deleted first. Therefore,
question_stats_cleanup_task is no more.
Tim Hunt