Prevent interface message from being used for sends

[helenktsai]

Issue transferred from internal repo

They should only be used for events (and technically only needed for multi inheritance) but sometimes get used for plain sends

Update core to:

• Obsolete IMessageCreator
• Obsolete the Send<T>(()=>...) overload
• Potentially provide a IEventCreator for users that still want to pre-create their events instead of using Publish<T>(()=>...)