ZMySQLDA: set time_zone session variable to UTC

Our patched MySQLDA assumes that DATE and DATETIME columns are in UTC,
but the TIMESTAMP columns are using mysql's time_zone variable, which
defaults to mysql server system timezone.

Set time_zone session variable at the beginning of each connection so
that TIMESTAMP columns are also UTC.

product/ZMySQLDA/db.py

@@ -314,6 +314,7 @@ class DB(TM):
               error=True,
             )
       self.db = MySQLdb.connect(**self._kw_args)
+      self._query("SET time_zone='+00:00'")
 
     def tables(self, rdb=0,
                _care=('TABLE', 'VIEW')):