dbquery: MySQL 5.5.3+ autocommit fix

• Since MySQL 5.5.3 DDL statements are transactional, thus they were possibly causing locks with other operations even though these were performed over MyISAM tables. This appears to be due to MySQLdb Python module that since version 1.2.0 disables autocommit by default. To avoid these deadlocks, it is enough to re-enable autocommit.

• This commit also revokes the workaround implemented in 512ec7b59fc2694dbea0973f74dd0a388ea4b737.