MXS-1380 Parse UNIONs completely

Before this change, only db2.t2 was reported as table name for a
select like

    select * from db1.t1 union select * from db2.t2

With this change, db1.t1 and db2.t2 are reported.

query_classifier/test/maxscale.test

@@ -79,3 +79,5 @@ ROLLBACK TO id;
 RELEASE SAVEPOINT id;
 
 SELECT her FROM (SELECT @@server_id as her) as t WHERE her REGEXP '.*';
+
+select * from db1.t1 union select * from db2.t2