Set the maximum workers to create.

def set_max_workers(self, nb):
        """
        Set the maximum workers to create.
        """
        self.count_lock.acquire()
        self.shared['max_workers'] = nb
        if self.shared['workers'] > self.shared['max_workers']:
            self.kill(self.shared['workers'] - self.shared['max_workers'])
        self.count_lock.release()