模拟一个自动订房退房客户端，用户可以订房退房，若房间已经预定，那么不能被其他客户端再预定，只有用户退房后，房间可再被预定-Simulate an automatic reservation check out the client, the user can check reservation if the room has been scheduled, so other clients can not be re-scheduled, only the user after checking out the room can be booked