Support DATETIME(6) for ROW START, ROW END

MariaDB [test]> CREATE OR REPLACE TABLE t (

    ->     id INT UNSIGNED NOT NULL AUTO_INCREMENT,

    ->     label VARCHAR(50) NOT NULL,

    ->

    ->         PERIOD FOR SYSTEM_TIME (valid_from, valid_to),

    ->     valid_from DATETIME(6)

    ->         GENERATED ALWAYS AS ROW START,

    ->     valid_to DATETIME(6)

    ->         GENERATED ALWAYS AS ROW END,

    ->

    ->     PRIMARY KEY (id)

    -> )

    ->     WITH SYSTEM VERSIONING,

    ->     ENGINE InnoDB

    -> ;

Query OK, 0 rows affected (0.010 sec)

MariaDB [test]> INSERT INTO t (label) VALUES ('blah blah');

Query OK, 1 row affected (0.001 sec)

MariaDB [test]> SELECT * FROM t;

Empty set (0.000 sec)

MariaDB [test]> SELECT * FROM t FOR SYSTEM_TIME ALL;

+----+-----------+----------------------------+----------------------------+

| id | label     | valid_from                 | valid_to                   |

+----+-----------+----------------------------+----------------------------+

|  1 | blah blah | 2018-10-14 19:56:08.057624 | 0000-00-00 00:00:00.000000 |

+----+-----------+----------------------------+----------------------------+