[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.

pica-db-backup/config/backup_config.json

@@ -19,7 +19,7 @@
     "Type": "mysql",
     "Folder": "etherpad",
     "Cron": "0 1 * * *",
-    "Options": "--single-transaction",
+    "Options": "--single-transaction --quick --lock-tables=false",
     "Init-Backup": "1"
   },
   "etherpad-week": {