Created by: jacquesqiao

project: https://github.com/PaddlePaddle/Paddle/issues/9211 fix: https://github.com/PaddlePaddle/Paddle/issues/9556

1. fix some bugs of PrefetchVariable interface. client and server can communicate with each other using PrefetchVariable
2. fix some code style problem of cpplint.
3. add some more log to help debug.