the basic idea is to change clean up procedure to 2 steps,
the 1st step only marks the time controller as to be cleaned up,
the 2nd step executes in the timer thread and does the
actual job to avoid race.

I also change `pCtrl->ticks = rand() / pCtrl->maxTicks` to
`pCtrl->ticks = rand() % pCtrl->maxTicks`, because I think
this is a typo but not sure about this.