journal: add a journal origin client to schedule loader tasks?

To separate concerns, the journal could be leverage to schedule loading tasks:

|---------------------------------------------------------------+-----------------------------------------------------------------------------|
| pros                                                          | cons                                                                        |
|---------------------------------------------------------------+-----------------------------------------------------------------------------|
| - separation of concerns (lister lists and creates origins,   | - the task's policy (recurring, oneshot) is not part of the journal's event |
| no longer schedules)                                          | (that might be triggering a schema migration on the scheduler's part: new   |
| - centralize the scheduling logic within that journal client  | origin_type column in task_type table)                                      |
| (that could simplify maintenance and be a step towards swh/devel/swh-scheduler#1157) | - add a new cog on swh machinery                                            |
|---------------------------------------------------------------+-----------------------------------------------------------------------------|

Related swh-scheduler#1157 (closed)

---

Migrated from T1471 (view on Phabricator)