Clarify uniqueness constraints on UIK IDs about mls-protocol

[beurdouche]

The current text says:

UserInitKeys also contain an identifier chosen by the client, which the client
MUST assure uniquely identifies a given UserInitKey object among the
set of UserInitKeys created by this client.

Would it be realistic to say that the Delivery Service MUST check when storing a key that the UIK identifier is not used by another key ? @bifurcation @raphaelrobert

[raphaelrobert]

I'd be fine with that requirement for the DS.

[bifurcation]

Given that arch PR and the fact that the doc already says "the client MUST assure uniquely identifies a given UserInitKey object among the set of UserInitKeys created by this client", I think we can call this fixed.