异步模型中HttpWebRequest的Timeout

要控制HttpWebRequest在10秒内timeout, 设置两个属性即可，Timeout, ReadWriteTime

但在异步模型中，.net framework却没有帮我们实现timeout.

http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.begingetresponse.aspx

如链接中所讲，In the case of asynchronous requests, it is the responsibility of the client application to implement its own time-out mechanism.