Bug Fix: change unix_timestamp(<expr>) to return 0 when it can't convert expr to a date

[fulghum]

MySQL's implementation of unix_timestamp(<expr>) returns 0 when the expression can't be converted to a date and logs a warning. This PR changes Dolt to have the same behavior. It also includes a bug fix for ctx.ClearWarnings to correctly clear warnings.

mysql> select unix_timestamp(1577995200);
+----------------------------+
| unix_timestamp(1577995200) |
+----------------------------+
|                          0 |
+----------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> select unix_timestamp("jason");
+-------------------------+
| unix_timestamp("jason") |
+-------------------------+
|                0.000000 |
+-------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+-----------------------------------+
| Level   | Code | Message                           |
+---------+------+-----------------------------------+
| Warning | 1292 | Incorrect datetime value: 'jason' |
+---------+------+-----------------------------------+
1 row in set (0.00 sec)

[zachmu]

LGTM!