Get the block number with the given consensus hash.
        Return None if there is no such block.

def get_block_from_consensus( self, consensus_hash ):
        """
        Get the block number with the given consensus hash.
        Return None if there is no such block.
        """
        query = 'SELECT block_id FROM snapshots WHERE consensus_hash = ?;'
        args = (consensus_hash,)

        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['block_id']

        con.close()
        return res