线程池生命周期的几种状态：
* RUNNING: Accept new tasks and process queued tasks
* SHUTDOWN: Don't accept new tasks, but process queued tasks
* STOP: Don't accept new tasks, don't process queued tasks,
* and interrupt in-progress tasks
* TIDYING: All tasks have terminated, workerCount is zero,
* the thread transitioning to state TIDYING
* will run the terminated() hook method
* TERMINATED: terminated() has completed
RUNNING：可以接受新的task，并且可以处理排队中的task
SHUTDOWN：不在接受新的task，但是可以执行队列中的task
STOP：不在接受新的task，并且也不再处理队列中的task
TIDYING：所有task全部终止，workerCount为0，线程调用terminated的相关方法状态切换至TIDYING
TERMINATED：terminated方法执行结束