Fix race condition in CPU frequency set with job preemption.
When the preemptor job completed, it would notify the srun, which
would notify the slurmctld, which could resume a preempted job.
That preempted job could reset the CPU frequency before the
preemptor. This change has the slurmstepd resetting a job's
CPU frequency prior to notifying srun of completion, which
eliminates the race condition.
bug 1011