Notification of string changes

Author: Tim Hunt
MDL-75576 quiz/question statistics: don't expire by time
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.


http://git.moodle.org/gw?p=moodle.git;a=commit;h=118d3e855a3755a7c24c2927652101c853ead85b
http://github.com/moodle/moodle/commit/118d3e855a3755a7c24c2927652101c853ead85b

+ 4.0 en [taskquestionstatscleanupcron,core_admin]