Inconsistency between vote and transaction signing

[rhsimplex]

Logically, all consensus functionality is relegated to the consensus plugin. However vote signing is currently handled in core.py. Consider how a tx is signed -- a derived class function of AbstractConsensusRules in consensus.py called sign_transaction calls a function in util.py called sign_tx (phew!):

def sign_tx(transaction, private_key):
    """Sign a transaction

    A transaction signed with the `current_owner` corresponding private key.

    Args:
        transaction (dict): transaction to sign.
        private_key (str): base58 encoded private key to create a signature of the transaction.

    Returns:
        dict: transaction with the `signature` field included.

    """
    private_key = crypto.SigningKey(private_key)
    signature = private_key.sign(serialize(transaction))
    signed_transaction = transaction.copy()
    signed_transaction.update({'signature': signature.decode()})
    return signed_transaction

Contrast to this similar-looking function for casting a vote which is handled entirely in core.py:

    def vote(self, block, previous_block_id, decision, invalid_reason=None):
        """Cast your vote on the block given the previous_block_hash and the decision (valid/invalid)
        return the block to the updated in the database.

        Args:
            block (dict): Block to vote.
            previous_block_id (str): The id of the previous block.
            decision (bool): Whether the block is valid or invalid.
            invalid_reason (Optional[str]): Reason the block is invalid
        """

        vote = {
            'voting_for_block': block['id'],
            'previous_block': previous_block_id,
            'is_block_valid': decision,
            'invalid_reason': invalid_reason,
            'timestamp': util.timestamp()
        }

        vote_data = util.serialize(vote)
        signature = crypto.SigningKey(self.me_private).sign(vote_data).decode()

        vote_signed = {
            'node_pubkey': self.me,
            'signature': signature,
            'vote': vote
        }

        return vote_signed

The vote signing logic should also be handled by the consensus plugin. Votes, signatures, etc should all be signed in the same place to avoid duplicating code.

[r-marques]

I agree with this. Lets take this into account when doing the refactor #312

[rhsimplex]

Should be made irrelevant by #500