Add jobid/stepid to MESSAGE_TASK_EXIT to address race condition when

    a job step is cancelled, another is started immediately (before the 
    first one completely terminates) and ports are reused. 
    NOTE: This change requires that SLURM be updated on all nodes of the
    cluster at the same time. There will be no impact upon currently running
    jobs (they will ignore the jobid/stepid at the end of the message).