`stopRequested()` reads only atomic variables. It doesn't need a mutex
to do this.
This function is called in `waitIfStopRequested()`, which in turn
is called before execution of every job, and inside some jobs. With this
change the mutex inside `waitIfStopRequested` needs to be locked only in
very rare cases instead of every time.
Mariusz Glebocki
Wilson Snyder
Kamil Rakoczy