Don't clear scheduled tasks at startup and ensure that the pingback scheduler does not add multiple pingbacks as a result

[rtibbles]

Observed behavior

In line with the regular task queue, we clear scheduled tasks at startup.
#7951 changed this behaviour for the regular task queue, but not for the scheduled tasks.

Expected behavior

We should be able to persist scheduled tasks across server restarts.
The 'schedule_pingback` function should not queue a new ping back in the scheduler if one already exists.

User-facing consequences

If we implement scheduled tasks created by the user, they would be cleared every time the server restarted.