verification hash calculation: the previous verification should be included at the root level, the same level as other hashes

[rht]

Currently, the verification hash is composed of hash_sum(content_hash + metadata_hash + signature_hash + witness_hash). Where the previous verification hash is an input to calculate metadata_hash.

I think it should be instead hash_sum(previous_verification_hash + content_hash + metadata_hash + signature_hash + witness_hash).