fix(tasks): tasks queue is cleared on server start

[vkWeb]

Summary

When the kolibri server starts, we were clearing the tasks queue which lead us to lose the historical information of completed, failed or cancelled tasks.

This PR fixes the issue by not letting that happen.

Reviewer guidance

1. Complete some tasks or cancel running tasks.
2. Restart the server.
3. See the task manager.
4. The task list should have persisted.

References

Fixes #6255

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[vkWeb]

cc @jonboiser, @rtibbles for review.

[vkWeb]

I think we should clear the tasks from db when the user clears them from the frontend...? This will simplify frontend's logic as we'll just need to display jobs that get return from GET api/tasks/tasks (no filtering) and also it'll save us some memory.

If we move forward with the current implementation then jobs will persist on db forever.

[rtibbles]

This fixes the issue as stated.

[rtibbles]

Added a follow up issue to do the same for the task scheduler: #7954