It is not used by library for a while. It could be used to pass
unsupported extension frames to application, but its interface
requires library to buffer entire frame, which we'd like to avoid.
For unsupported extension frames, we will add new callbacks which does
not require buffering if they are required.
To make it possible to add new callbacks without bumping so name, we
decided to hide details of nghttp2_session_callbacks. We provide
setter like functions to set individual callback function.