delay for previous_key

[fimbault]

from @adeinega "am I right that previous_key becomes immediately invalid? If it's true, wouldn't be nice to have here some way to rotate them more or less gracefully? It could be useful when there are multiple running instances of the client and it's tough, or even impossible, to update the keys on all these instances immediately (just as an example, the client instance could be deployed as a service in K8S)."

We'll handle this via a (non normative) security consideration

[adeinega]

#457 looks good, thank both of you.

There was one thing more, an AS may not always allow its client instances to rotate the keys, correct? Therefore,

1. the client instances might "learn" whether it's possible in advance, during ongoing negotiation with the AS
2. the specification may define a dedicated error that will basically tell the client instances, this operation isn't allowed