When working against external services it can be advantageous to not
have automated tasks all hit the service at the exact same time from
many different installations.
This change allows the use of 'R' in hour/minute fields of the scheduled
task definition so a task will be installed with a 'random' hour/minute
value for these sort of occasions.
Note that the task will be installed in the database with a defined
time, this simply randomises the time chosen when loading from the
definiton.
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.
