MEDIUM: quic: strengthen MUX send notification

MEDIUM: quic: strengthen MUX send notification

Previous commit implement a refactor of MUX send notification from
quic_conn layer. With this new architecture, a proper callback is
defined for each qc_stream_desc instance.

This architecture change allows to simplify notification from quic_conn
layer. First, ensure the MUX callback to properly ignore retransmission
of an already emitted frame. Luckily, this can be handled easily by
comparing offsets and FIN status. Also, each QCS instance can now be
unregistered from send notification just prior qc_stream_desc releasing.
This ensures a QCS is never manipulated from quic_conn after its
emission ending. Both these changes render the send notification more
robust. As a nice effect, flag QUIC_FL_CONN_TX_MUX_CONTEXT can be
removed as it is now unneeded.