message: Change max connection id length to 20
  Summary: This also changed in draft-22, found during interop.
diff: namespace quic {
  constexpr size_t kMinConnectionIdSize = 4;
  -constexpr size_t kMaxConnectionIdSize = 18;
  +constexpr size_t kMaxConnectionIdSize = 20;
  // set conn id version at the first 4 bits
  constexpr uint8_t kShortVersionId = 0x1;
  