Get the list of valid consensus hashes for a given block.

def get_valid_consensus_hashes( self, block_id ):
        """
        Get the list of valid consensus hashes for a given block.
        """
        first_block_to_check = block_id - self.impl.get_valid_transaction_window()

        query = 'SELECT consensus_hash FROM snapshots WHERE block_id >= ? AND block_id <= ?;'
        args = (first_block_to_check,block_id)

        valid_consensus_hashes = []
        
        con = self.db_open(self.impl, self.working_dir)
        rows = self.db_query_execute(con, query, args, verbose=False)

        for r in rows:
            assert r['consensus_hash'] is not None
            assert isinstance(r['consensus_hash'], (str,unicode))

            valid_consensus_hashes.append(str(r['consensus_hash']))

        con.close()
        return valid_consensus_hashes