save one PacketHeader copy in QuicHeaderCodec
save one PacketHeader copy in QuicHeaderCodec

Summary: create the temp obj inline

Reviewed By: vchynarov

Differential Revision: D17397927

fbshipit-source-id: 20757bf61260d6508ef514b68b6bbcdaac0f95a8

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst