BUG/MINOR: mux-quic: ensure close-spread-time is properly applied

BUG/MINOR: mux-quic: ensure close-spread-time is properly applied

If a connection remains on a proxy currently disabled or stopped, a
special spread timeout is set if active close is configured. For QUIC
MUX, this is set via qcc_refresh_timeout() as with all other timeout
values.

Fix this closing timeout setting : it is now used as an override to any
other timeout that may have been chosen if calculated spread time is
lower than the previously selected value. This is done for backend
connections as well.

This should be backported up to 2.6 after a period of observation.