[refactor] extract the code that handles PRIORITY_UPDATE to a dedicated function
[refactor] extract the code that handles PRIORITY_UPDATE to a dedicated function

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