[http3client] set `write_req` callback after the stream is open
[http3client] set `write_req` callback after the stream is open

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