search for: nbd_aio_pread_structured_allback

This has an annoying subtlety around the fact that we can pass a single user_data and multiple closures in one function call. The LIBNBD_CALLBACK_FREE function would be called several times with the same user_data in this case, which means the callback must do some kind of reference counting before the user_data can be freed. I propose that we split up Closure so it describes a single closure,