Kill all active workers.
        @wait: Seconds to wait until last worker ends.
               If None it waits forever.

def killall(self, wait = None):
        """
        Kill all active workers.
        @wait: Seconds to wait until last worker ends.
               If None it waits forever.
        """
        self.queue.close()
        self.count_lock.acquire()
        self.kill(self.shared['workers'])
        self.count_lock.release()
        self.tasks.close()
        self.terminate()
        self.kill_event.wait(wait)