MINOR: mux_quic/pacing: display pacing info on show quic

MINOR: mux_quic/pacing: display pacing info on show quic

To improve debugging, extend "show quic" output to report if pacing is
activated on a connection. Two values will be displayed for pacing :

* a new counter paced_sent_ctr is defined in QCC structure. It will be
  incremented each time an emission is interrupted due to pacing.

* pacing engine now saves the number of datagrams sent in the last paced
  emission. This will be helpful to ensure burst parameter is valid.