BUG/MINOR: mux_quic: refresh timeout only if I/O performed

BUG/MINOR: mux_quic: refresh timeout only if I/O performed

Previously, QUIC MUX timeout was refreshed on every qcc_io_cb()
execution. This is not desired if no send/receive were performed, as in
this case the connection may be stuck.

This patch fixes this by refreshing timeout only if some progress is
performed during qcc_io_cb(). To implement this, return value of
qcc_io_recv() has been adjusted to return the number of newly decoded
bytes.

This patch is considered as a bug fix as without it there is a risk of
QUIC MUX inactivity timeout to be less efficient and to maintain a
connection too long.

This should be backported up to 2.8, after a period of observation.