Fix issue with multiple jobs from an array to start.

By removing the real locks we can get into a race condition where the prolog
starts and finishes before we get here and then we end up waiting forever.

Making the mutex a static seemed to help in many cases, but didn't
completely close the window.  Changing slurm_cond_wait to
slurm_cond_timedwait fixed the scenario where we would hit the window, but
not degrade performance the original commit provides.

There were also spots where if the job or step didn't exist it wouldn't
signal the conditional also providing a spot this could get stuck not
starting the job.

Fix regression from commit 52ce3ff0

Bug 3977