[Reviewer: Rob] Iss505

Rob, can you review this change to add the subscription state header in. This also fixes a bug where the wrong subscription state is reported for actively expired subscriptions in the NOTIFY
This is a bit of a nasty type and it took me a while to figure out how it was used. Please can you add a comment explaining that its overloaded to cover three separate things.

FTR I think a better way to handle this is with separate enums for `RegState`, `ContactState` and `SubscriptionState`.