Change max connection id length to 20
Change max connection id length to 20

Summary: This also changed in draft-22, found during interop.

Reviewed By: JunqiWang

Differential Revision: D16344998

fbshipit-source-id: 5eb1afa941661ef02595cc95d016421bbcdc568f

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