MINOR: mux-quic: increment pacing retry counter on expired

MINOR: mux-quic: increment pacing retry counter on expired

A field <paced_sent_ctr> from quic_pacer structure is used to report the
number of occurences where emission has been interrupted due to pacing.
However, it was not incremented when QUIC MUX had to pause immediately
emission as pacing was still not yet expired.

Fix this by incrementing <paced_sent_ctr> in qcc_io_send() prior to
emission if pacing is expired. Note that incrementation is only done
once if the tasklet is then repeatdely woken up until the timer is
expired.

This should be backported up to 3.1.