BUG/MEDIUM: mux_quic: do not reset timeout after shut

BUG/MEDIUM: mux_quic: do not reset timeout after shut

When a connection is shutting down, MUX QUIC uses shut-timeout if
defined as a final timeout value. However, the timer was reset every
time qcc_refresh_timeout() was called. In effect, a connection could
stay indefinitely due to repeated activity from the peer.

Fix this by ensuring shut-timeout is only set once in
qcc_refresh_timeout(). The value is not updated if QCC task timeout is
already set.

Note that shut-timeout could be completely ignored if set to a greater
value than the default client/server timeout. However, as the docs
imply, this is not expected, as it is used to ensure a closing
connection will quickly end.

This must be backported up to 2.8.