And don't add/resend it multiple times. 1. We can be in a setup with one SV only and no AddSignature() called at all. 2. AddSignature() might add M-1 signatures and our signature should be the last one to complete MPTRoot, but we'll never do that.
Stateroot payload must have it.
Match C# implementation. It's easier to do this by changing the structure itself.
It's only needed to verify/sign.
