[pg] ignore from pg_settings in set_config queries

[qrort]

Rewrite queries such as SELECT set_config(\'bytea_output\',\'hex\',false) FROM pg_settings WHERE name = \'bytea_output\'; into SELECT set_config(\'bytea_output\',\'hex\',false) if FROM clause contains pg_settings table.

[qrort]

In the future we could force a query to execute as single stage. In such cases, we can execute SELECT set_config(\'bytea_output\',\'hex\',false) FROM pg_settings WHERE name = \'bytea_output\';