DRILL-1779: Add unix_timestamp function.

Support unix_timestamp and unix_timestamp(date) as described here:
http://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html#function_unix-timestamp

unix_timestamp wil return the number of milliseconds since epoch.
unix_timestamp(<date>) will return the number of milliseconds between epoch
and the given date. date should be given as a string literal within single
quote.

Testing:
0: jdbc:drill:zk=local> select unix_timestamp() from cp.`tpch/nation.parquet`
limit 1;
+------------+
|   EXPR$0   |
+------------+
| 1416959178094 |
+------------+

0: jdbc:drill:zk=local> select unix_timestamp('2014-11-25 15:07:00.234') from
cp.`tpch/nation.parquet` limit 1;
+------------+
|   EXPR$0   |
+------------+
| 1416928020234 |
+------------+