java实现的线程池，可以在指定时间间隔内有只有一个任务执行，同时也可以设定在线程执行结束后释放的时间。

/**

• This thread pool will keep only one runnable is running at given time interval.
• And if no task is to be executed and has been waiting for given aliveTimeMs time,
• execute worker will be released. */