docs: public Scheduler stop(boolean clearTasks) {
  		synchronized (lock) {
  			if (false == started) {
  				throw new IllegalStateException("Scheduler not started !");
  			}
  
  			// 停止CronTimer
  			this.timer.stopTimer();
  			this.timer = null;
  			
  			//停止线程池
  			this.threadExecutor.shutdown();
  			this.threadExecutor = null;
  			
  			//可选是否清空任务表
  			if(clearTasks) {
  				clear();
  			}
  
  			// 修改标志
  			started = false;
  		}
  		return this;
  	}