Add functions to convert custom date strings with full precision

Impala can only convert custom-formatted date strings to a bigint of seconds (using UNIX_TIMESTAMP and FROM_UNIXTIME), which removes the fraction of the second from the result. This loss of precision does not meet the requirements of some applications.

Impala could add functions to convert these strings directly to a timestamp and vice versa, for example:

• to_date(date timestamp, format string), return type string
• from_date(date string, format string), return type timestamp