Use consistent names in the main sequence diagram

Previously the diagram had the concept of (for example) "Sending user". That isn't wrong, but it is also not consistent on how we talk about that user in the remainder of the spec. Now it is called the "Originator user". Same for the other actors/entities.

I took the liberty to change two more things; I renamed 'KYC on payment address' to 'on chain analytics on payment address' and made the text in the protocol arrows between the VASPs more clear.