Only pass MessageType as string to ISubscriptionStorage

[andreasohlund]

Given that we now ignore the version we can simplify ISubscriptionStorage.X to:

 public interface ISubscriptionStorage
    {
        Task Subscribe(Subscriber subscriber, string messageType, ContextBag context);

        Task Unsubscribe(Subscriber subscriber, string messageType, ContextBag context);

        Task<IEnumerable<Subscriber>> GetSubscriberAddressesForMessage(IEnumerable<string> messageTypes, ContextBag context);
    }

we can also remove the MessageType class in the core since it adds no value.