fix(scheduler): scheduled tasks does not persist on server restart

[vkWeb]

Summary

On #7951 we changed the behaviour for the regular task queue to stay persistent on server restarts, but not for the scheduled tasks. This PR achieves persistency for scheduled tasks.

• Now, we should be able to persist scheduled tasks across server restarts.
• The schedule_pingback and schedule_vaccum functions should not queue in the scheduler if one already exists.

References

Closes #7954

Reviewer guidance

1. Clear all the records of scheduled_jobs table in job_storage.sqlite3 db to avoid any confusion during testing.
2. Start the server, now check the scheduled_jobs table, two jobs must be scheduled: _ping and vacuum.
3. Schedule a new job.
4. Restart the server, now all the scheduled jobs (_ping, vacuum and your own job) must have persisted.
5. Also, make sure that _ping and vacuum aren't queued again on server restarts.

---

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[vkWeb]

cc @rtibbles for review.

[rtibbles]

Can simplify approach and do some cleanup.

Would also be a good to add some tests of this behaviour.

[vkWeb]

@rtibbles sir approach has been simplified as you suggested. I'm new to mock so I'm learning it, tests will be pushed by tomorrow.

[rtibbles]

This works as expected in local testing, and has the added benefit of not repeating the ping back on repeated restarts of the server - it persists the previous success and will ping again when the time is right!