switch to using 3xmaxPTO between key updates

[kazuho]

Closes #209.

[huitema]

Please specify the "PTO" a bit more precisely.

[marten-seemann]

This works, but it’s additional complexity compared to RFC 9001. #215 proposes a different solution that doesn’t require us to do anything differently.

[kazuho]

@marten-seemann I do not think changing 3PTO to 3maxPTO is a great deal. For existing QUIC stacks using three keys and PN in conjunction, switching to two keys would be a simplification.

Compared to that, it is my opinion that using different keys on each path and running key updates separately as proposed in #215 is an unnecessary complexity.

[kazuho]

Take it or leave it, but we might want to clarify that this value is different from the recommendation that we have in QUIC version 1.

[huitema]

I like the edited version, and I agree with the suggested change.

[mirjak]

I think it's simpler than the current text, therefore I think we should merge it for now (and potentially keep discussing further changes later).

[Yanmei-Liu]

It's very clear. Looks good to me.