[suggestion] Dedup public keys in transaction.signature and transaction.payload.authority

[s8sato]

As of f12077d there is a duplication between transaction.signature.public_key and transaction.payload.authority.signatory.
The former could also serve as the latter role, but the problem is that authority: AccountId is required for subsequent process and it consists of not only public key but also DomainId which cannot be known from transaction.signature.
The solution would be to drive DomainId out of AccountId, but the main blocker is that AccountId should inform of its parent domain for event scoping.
So the first step would be to retrieve DomainId somewhere other than AccountId at the account event emission