Created by: gongweibao

Add an rpc interface get_config on serving. So

1. client needs not to prepare the model config file.
2. client can get the max_batch_size from this server.

message ServingConfig{
    int32 max_batch_size = 1;
    string proto_txt = 2;
};

service MultiLangGeneralModelService {	
  rpc get_config(EmptyRequest) returns (ServingConfig) {}
};