[http3] Use QUIC packet type macro constants
[http3] Use QUIC packet type macro constants

This patch replaces raw hex numbers for QUIC packet types with macro
constants defined in quicly.

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