[http3] Count conns_accepting alone for quic_num_connections
[http3] Count conns_accepting alone for quic_num_connections

Because a connection is always registered to both `conns_by_id` and
`conns_accepting` throughout its lifetime, counting `conns_by_id`
is enough to know the total number of connections.

yannick/h2o,devnexen/h2o,h2o/h2o,yannick/h2o,cwyang/h2o,h2o/h2o,devnexen/h2o,cwyang/h2o,h2o/h2o,cwyang/h2o,yannick/h2o,devnexen/h2o,cwyang/h2o,h2o/h2o,h2o/h2o,cwyang/h2o,yannick/h2o,devnexen/h2o,h2o/h2o,devnexen/h2o,yannick/h2o,devnexen/h2o,devnexen/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,cwyang/h2o,yannick/h2o,devnexen/h2o,cwyang/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,h2o/h2o,h2o/h2o