MINOR: mux-quic: measure QCS lifetime and its blocking state

MINOR: mux-quic: measure QCS lifetime and its blocking state

Reuse newly defined tot_time structure to measure various values related
to a QCS lifetime.

First, a timer is used to comptabilize the total QCS lifetime. Then, two
other timers are used to account the total time during which Tx from
stream layer to MUX is blocked, either on lack of buffer or due to
flow-control.

These three timers are reported in qmux_dump_qcs_info(). Thus, they are
available in traces and for QUIC MUX debug string sample.