A worker must be uniquely identified.
A task in status "in progress" must reference the id of related worker through the new column worker_uuid (VARCHAR(40)) to tables CE_QUEUE and CE_ACTIVITY.
- a newly created task (ie. with status "PENDING") has no worker UUID
- a task "IN PROGRESS" always has a worker UUID
- a task "PENDING" may have a UUID if it has already been executed at least once
- tasks created in this table prior to this ticket do not have a worker uuid
- a "CANCELED" task may or may not have a worker uuid (depending on whether it has been executed at least once or not)
- tasks with status "FAILED" and "SUCCESS" have a worker uuid