Try to avoid copying QuicDatagrams data

[ddragana]

The copy that happens here is a little unfortunate. It might be a good idea to mark this for future investigation. I don't think that a copy is going to be so bad for us, but we might want to enable a more efficient handling process. That's a project though, so this is fine.

Originally posted by @martinthomson in https://github.com/mozilla/neqo/pull/1201#discussion_r689962436