The API we're calling requires us to pass an absolute point in time as an
argument (`pthread_cond_timedwait`) so we call `gettimeofday` ahead of time to
then add the specified duration to. Unfortuantely the current "add the duration"
logic forgot to take into account the current time's sub-second precision (e.g.
the `tv_usec` field was ignored), causing sub-second duration waits to return
spuriously.