MINOR: quic: redirect app_ops snd_buf through mux

MINOR: quic: redirect app_ops snd_buf through mux

This change is required to be able to use multiple app_ops layer on top
of QUIC. The stream-interface will now call the mux snd_buf which is
just a proxy to the app_ops snd_buf function.

The architecture may be simplified in the structure to install the
app_ops on the stream_interface and avoid the detour via the mux layer
on the sending path.