Add full support for Hour/Minute/Second to UNIX_TIMESTAMP() function

MariaDB [test]> create table cs2 (a decimal(18))engine=columnstore;

Query OK, 0 rows affected (0.242 sec)

MariaDB [test]> insert into cs2 values (20201028183200);

Query OK, 1 row affected (0.237 sec)

MariaDB [test]> select unix_timestamp(a) from cs2;

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

| unix_timestamp(a) |

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

|        1603857600 |

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

1 row in set (0.151 sec)

MariaDB [test]> create table i2 as select * from cs2;

Query OK, 1 row affected (0.047 sec)

Records: 1  Duplicates: 0  Warnings: 0

MariaDB [test]> select unix_timestamp(a) from i2;

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

| unix_timestamp(a) |

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

|        1603924320 |

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

1 row in set (0.002 sec)