MEDIUM: mux-quic: implement QMux send

MEDIUM: mux-quic: implement QMux send

This patchs implement mux-quic reception for the new QMux protocol. This
is performed via the new function qcc_qstrm_send_frames(). Its interface
is similar to the QUIC equivalent : it takes a list of frames and
encodes them in a buffer before sending it via snd_buf.

Contrary to QUIC, a check on CO_FL_ERROR flag is performed prior to
every qcc_qstrm_send_frames() invokation to interrupt emission. This is
necessary as the transport layer may set it during snd_buf. This is not
the case currently for quic_conn layer, but maybe a similar mechanism
should be implemented as well for QUIC in the future.