本文目标是：
- 弄清updateBlockForPipeline这个RPC的作用。
- 弄清updateBlockForPipeline RPC的使用场景，代码里的调用点。
一、updateBlockForPipeline的作用
其定义在ClientProtocol接口里，是Client与NameNode之间的接口。
看其代码注释描述：
为一个under construction状态下的block获取一个新的GS与access token。
此RPC只有在client需要恢复一个失败的pipeline时、或者为了append建立一个pipeline时会被调用。
/**
* Get a new generation stamp together with an access token for
* a block under construction
*
* This method is called only when a client needs to recover a failed
* pipeline or set up a pipeline for appending to a block.
*
* @param block a block
* @param clientName the name of the client
* @return a located block with a new generation stamp and an access token
* @throws IOException if any error occurs
*/
