Created by: Yancey1989

From the comment: https://github.com/PaddlePaddle/Paddle/pull/10550#discussion_r191110767 , and https://github.com/PaddlePaddle/Paddle/pull/10550#discussion_r191110950

We need to keep variable and gRPC client thread-safe.

After the offline discuss with @typhoonzero, we can make RPCClient as a singleton mode, and don't need to change variable implement anyway.