change the open method in ClientProtocol to take an additional argument: clientMachine

The open method returns a list of blocks that belong to a file and, for each block, a list of datanodes that contain the block.To improve reading efficiency, I'd like to return the list of datanodes in the order of its distance to the client machine. So I suggest that we make the open method to take an additional argument clientMachine which is in the format of hostname:port#.