@Override
                        public Connection getConnection() {
                            if (conn == null) {
                                throw new IllegalStateException("fail to get connection");
                            }
                            return conn;
                        }
GRPC RPC server implement.

@author nicholas.jxf
@author jiachun.fjc
jraft-extension/rpc-grpc-impl/src/main/java/com/alipay/sofa/jraft/rpc/impl/GrpcServer.java