[TLS] DTLS 1.3 ACKs

[Eric Rescorla <[e-mail]>]

Hi folks,

https://github.com/tlswg/dtls13-spec/pull/9

I have just posted a PR to implement ACKs, largely as we discussed in
Chicago.
Here's the pull comment:

I have taken a slightly different approach here than we discussed
in Chicago. Specifically:

   -

   ACKs are not handshake messages but a new content type. This
   avoids the need to ack ACKs or worry about reassembling
   when ACKs are lost.
   -

   The receipt of a flight also counts as an implicit ACK and
   so you shouldn't ACK full flights. I tried removing this
   feature but it leads to weird anomalies where you have
   two flights you should be retransmitting if the ACKs get
   lost.
   -

   ACKs are always encrypted with the then-current key.
   -

   I also removed the rule that you don't stop retransmitting
   when you receive a partial flight. That was intended to have
   fast retransmission for missing messages because the retransmit
   of flight A triggered retransmission of partial flight B, but
   ACKs serve this purpose

Comments welcome.

-Ekr