[noise] Resumption PSKs

[David Wong]

> The issue is that the transport keys and these new "key chains" have
> to be created at the same time, so that the value used to derive them
> (the chaining key ck) can be deleted afterwards.

I don't really like that Split() returns several values that will
_most of the time_ not be useful to developers of protocols.
I don't think these need to be created at the same time, you could
iterate the chaining key one more time after Split() and keep that
one. Then an export API could use this new chaining key to generate
secrets.

David