Subject: Multiple waits on the same CV

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Be sure to LOCK and UNLOCK the associated mutex before
and after doing any pthread_cond_(timed)wait()s.

The man page says that doing so produces results that
are "undefined".  I think it is particularly problematic
when TWO threads try to do pthread_timed_wait() on a
single mutex at roughly the same time.

One student attempted this and that student's rps_timer()
function went into a permanent slumber. As soon as I
put the mutex_lock/unlock around the timed_wait it
worked fine.