grpc++不支持异步多次写入
(金庆的专栏 2017.5)
见：
Multi ClientAsyncWriter::Write() calls will crash. #7659
https://github.com/grpc/grpc/issues/7659
why the continuous write to async streaming crash? #4007
https://github.com/grpc/grpc/issues/4007
write stream error:GRPC_CALL_ERROR_ALREADY_INVOKED
https://github.com/grpc/grpc/issues/3953
grpc_cb 支持真正的异步写。
https://github.com/jinq0123/grpc_cb