MINOR: mux-quic: adjust timeout to accelerate closing

MINOR: mux-quic: adjust timeout to accelerate closing

Improve timeout handling on the MUX. When releasing a stream, first
check if the connection can be considered as dead and should be freed
immediatly. This allows to liberate resources faster when possible.

If the connection is still active, ensure there is no attached
conn-stream before scheduling the timeout. To do this, add a nb_cs field
in the qcc structure.