NEL cache should be partitioned

[yoavweiss]

#136 and the related paper point out an issue with the fact that the NEL cache is not partitioned. While in practice it is partitioned in Chromium, we should fix the spec to reflect that.

[clelland]

@jyasskin suggested that we can use the storage key concept from https://github.com/whatwg/storage as a partition key in this spec.

I think we'd need a note that implementers can use stronger keys, but that should set a minimum bar. And then if Storage adopts double- or triple-keying, NEL should get that for free at that point.

[annevk]

Shouldn't it use the network partition key?

[clelland]

Is that concept defined in a spec? I can imagine that it would be a better fit, but I'm not certain

[annevk]

Yes, in Fetch.