Return the consensus hash at a block number.
        Return {'status': True, 'consensus': ...} on success
        Return {'error': ...} on error

def rpc_get_consensus_at( self, block_id, **con_info ):
        """
        Return the consensus hash at a block number.
        Return {'status': True, 'consensus': ...} on success
        Return {'error': ...} on error
        """
        if not check_block(block_id):
            return {'error': 'Invalid block height', 'http_status': 400}

        db = get_db_state(self.working_dir)
        consensus = db.get_consensus_at( block_id )
        db.close()
        return self.success_response( {'consensus': consensus} )