BUG/MEDIUM: quic: ensure quic-conn survives to the MUX

BUG/MEDIUM: quic: ensure quic-conn survives to the MUX

Rationalize the lifetime of the quic-conn regarding with the MUX. The
quic-conn must not be freed if the MUX is still allocated.

This simplify the MUX code when accessing the quic-conn and removed
possible segfaults.

To implement this, if the quic-conn timer expired, the quic-conn is
released only if the MUX is not allocated. Else, the quic-conn is
flagged with QUIC_FL_CONN_EXP_TIMER. The MUX is then responsible
to call quic_close() which will free the flagged quic-conn.