Wrong default timezoneId for timestamps with no timezoneid

[marcosapmf]

Hi!
When I try to execute a SELECT query with a timestamp with no time zone id, it returns a different timestamp. For intance:
SELECT TIMESTAMP '2019-02-11 20:26:53.218'; returns 2019-02-12 04:26:53.218+00 as result.

This happens because of the default time zone id google sets is America/Los_Angeles (GMT-8) (see google/zetasql#133). However, the default timezoneid can be set but set_default_timezone_id is not wrapped to be called on Go side.