Galera cluster support
Galera cluster provides multi master SQL database (e.g. upon MariaDB or MySQL) and can help strengthen the database backend. As it easy to deal with mattermost-server outages (simply run another instance very quickly), it is mostly painfull to deal with single DB outage.
Galera cluster can be supported in a direct way - add all db nodes to the configuration and use it in balanced way or as a fallback - or can be used via haproxy or similar.
I think this is not need... and I believe it's not in Mattermost's scope.
You can install haproxy near mattermost and have it configured to be aware of the Galera Cluster.
What Mattermost still need to do... is keep it compatibility w/ MariaDB/MySQL.