[DBBackup] Try to improve performance for MySQL

See https://stackoverflow.com/a/12142501
Notice : this option can affect consistency of data for MyISAM tables, such as Etherpad store.
But this will avoid a 30 min full lock of the service, and these inconsistencies will be nothing if we really need a backup to restore the database ; we can afford to lose a few revisions and we can also recover a pad content because of the duplications in the table. The consistency is not that important for a pseudo key-value store such as the one that Etherpad uses.