MINOR: quic: refactor MUX send notification

MINOR: quic: refactor MUX send notification

For STREAM emission, MUX QUIC generates one or several frames and emit
them via qc_send_mux(). Lower layer may use them as-is, or split them to
lower chunk to fit in a QUIC packet. It is then responsible to notify
the MUX to report the amount of data sent.

Previously, this was done via a direct call from quic_conn to MUX using
qcc_streams_sent_done(). Modify this to have a better isolation accross
layers. Define a send callback handled by the qc_stream_desc instance.
This allows the MUX to register each QCS instance individually to the
renamved qmux_ctrl_send() which replaces qcc_streams_sent_done().

At quic_conn layer, qc_stream_desc_send() can be used now. This is a
wrapper to qc_stream_desc layer to invoke the send callback if
registered.

This mechanism of qc_stream_desc callback should be extended later to
implement other notifications accross the QUIC stack.