Remove node from load balancing when MariaDB node is not synced

When MariaDB is down on a node, it should not be load balanced to. For nodes running a web server as well as MariaDB, this can be achieved using the load_balancer_health_checks_groups_pairs attribute.

Currently, nodes are not removed from load balancing on nodes on which MariaDB is not down but not functional either because Galera is not in the state 'synced'. E.g. after restarting MariaDB. This causes WSREP has not yet prepared node for application use errors.