message: Remove temporary appender from encodeShortHeader.
  Summary:
  There's no reason to make a new one here, as there's one that's a member of the builder.
  This ends up actually using .4% CPU in a tperf test.
diff: @@ -239,22 +239,23 @@ void RegularQuicPacketBuilder::encodeShortHeader(
  remainingBytes_ = 0;
  return;
  }
  - folly::io::QueueAppender appender(&header_, kAppenderGrowthSize);
  uint8_t initialByte =
  ShortHeader::kFixedBitMask | (packetNumberEncoding_->length - 1);
  initialByte &= ~ShortHeader::kReservedBitsMask;
  if (shortHeader.getProtectionType() == ProtectionType::KeyPhaseOne) {
  initialByte |= ShortHeader::kKeyPhaseMask;
  }
  - appender.writeBE<uint8_t>(initialByte);
  + headerAppender_.writeBE<uint8_t>(initialByte);
  --remainingBytes_;
  - appender.push(
  + headerAppender_.push(
  shortHeader.getConnectionId().data(),
  shortHeader.getConnectionId().size());
  remainingBytes_ -= shortHeader.getConnectionId().size();
  appendBytes(
  - appender, packetNumberEncoding_->result, packetNumberEncoding_->length);
  + headerAppender_,
  + packetNumberEncoding_->result,
  + packetNumberEncoding_->length);
  }
  void RegularQuicPacketBuilder::push(const uint8_t* data, size_t len) {
  