Get the consensus hash at a given block.
        Return the consensus hash if we have one for this block.
        Return None if we don't

def get_consensus_at(self, block_id):
        """
        Get the consensus hash at a given block.
        Return the consensus hash if we have one for this block.
        Return None if we don't
        """
        query = 'SELECT consensus_hash FROM snapshots WHERE block_id = ?;'
        args = (block_id,)

        con = self.db_open(self.impl, self.working_dir)
        rows = self.db_query_execute(con, query, args, verbose=False)
        res = None

        for r in rows:
            res = r['consensus_hash']

        con.close()
        return res