swh-scheduler: Document the design of the scheduler in its documentation
It's an unavoidable piece of software for scheduling oneshot and recurring tasks.
It has a number of cogs to describe for the old data-model task (still in use for save-code-now & listers):
- listener
- runner (lister, deposit check, deposit load tasks)
- runner priority (save-code-now tasks)
- schedule-recurrent (all recurrent origins to visit)
Migrated from T3684 (view on Phabricator)
Edited by Antoine R. Dumont