Add pending or submitted status to TaskStatus

If we persist with the task first, then according to the specification, this status will be working. If persistence succeeds, but all subsequent calls fail, then this task will remain in the working state in the database. The next time the client resumes accessing tasks/list, it will see this task, but it hasn't actually been executed.

A possible solution is to not set a status value during task persistence, equivalent to a NULL state, and only update the status to working when the task is correctly triggered.

@LucaButBoring