Add periodic task to check the status of jobs

[nokome]

The overseer listens for job and worker events and notifies the manager. But it seems that sometimes, somehow, job events are lost. For example, some archive jobs seem to go forever but the new bucket is created and the callback is never called. It may make sense to have a periodic tasks (run by assistant) that checks jobs, including session jobs, and call AsyncResult on it get it's status etc.