`HiveResult.toHiveString` does not use the current session time zone

Currently, date/timestamp formatters in `HiveResult.toHiveString` are initialized once on instantiation of the `HiveResult` object, and pick up the session time zone. If the sessions time zone is changed, the formatters still use the previous one.

See the discussion there https://github.com/apache/spark/pull/23391#discussion_r397347820