Use mariadb for backing up the database

[tio-trom]

Currently we use ynh_mysql_dump_db and we put friendica into maintenance before doing that, else for large databases it will fail to backup. However that means if you do daily backups you'll have your network in maintenance mode for 30 min to an hour or more a day. This is not good.

But we can backup with mariadb that does incremental backups - https://mariadb.com/kb/en/full-backup-and-restore-with-mariabackup/

However I do not know how we can do it through YNH.

[tio-trom]

Closing in favor of https://github.com/YunoHost-Apps/friendica_ynh/issues/111