In the be8f453b74 the gradelib loading was removed by mistake. So
the library may not be loaded and executing the task may end with
the exception - class 'grade_grade' not found.
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.