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.
It's been a mistake to make every Moodle site out there to check for
available update every two hours, on the hour. We are experiencing
server load peaks that are harder and harder to deal with.
The patch addresses this by (1) checking for available updates just once
a day and (2) distributing the checking times randomly around the clock.
The mark started function for the completions scheduled task can be very
slow and causes performance issues when running. By splitting this into
it's own task it can be managed independantly from the other parts which
update regularly.
Centralise management of all types of cron tasks with registration, scheduling,
parallel task conflicts(blocking) and running once off tasks, all using an
administration screen.
This is a combination of several issues:
MDL-25502: Added "black magic" task allocator for cron
MDL-25503: Add step to cron to run all scheduled tasks
MDL-25504 cron: Refactor to use scheduler
MDL-25505: Add an admin interface to schedule tasks via cron.
MDL-25507: Add support for adhoc tasks.
