Created by: gongweibao

• Add rpc_server rpc_client interface to decouple RPC communication and operators
  • Move sync_mode_ scope_ from grpc server to operators which should care about them.
  • Add some comments about the RPC client interface.
  • Cleanup the source directory.
  • Omit the argument ctx in functions such as AsyncSendVariable?
  • Cleanup deserialization.
• Add brpc support
  • Add brpc framework.
  • Comptable with grpc::ByteBuffer and brpc::IOBuf
  • Add brpc implementation of rpc_server rpc_client.
  • Add brpc unit test.
  • use fewer channel and more stub
  • How to prevent many client access one server at same time.
  • Clean up var_is_not_stable
  • Modify rdma from macro to if condition.
• Check tensor size to avoid small data transfermation.