Sets and returns _WAL_NAME to 'wal' or 'xlog' depending on
        version of postgres we are working with.

        It is used for handling xlog -> wal rename in postgres v10

def _wal_name(cls):
        """
        Sets and returns _WAL_NAME to 'wal' or 'xlog' depending on
        version of postgres we are working with.

        It is used for handling xlog -> wal rename in postgres v10

        """
        if cls._WAL_NAME is None:
            version = cls._dict_transform(psql_csv_run(
                    "SELECT current_setting('server_version_num')"))
            if int(version['current_setting']) >= 100000:
                cls._WAL_NAME = 'wal'
            else:
                cls._WAL_NAME = 'xlog'
        return cls._WAL_NAME