Create option to configure DONOR/DESYNCED Galera nodes to be unavailable for load-balancing

When a node is in the DONOR/DESYNCED state, it doesn't participate in flow control, so its data can get stale. Galera states are explained here:

http://galeracluster.com/documentation-webpages/nodestates.html#changes-in-the-node-state

A node's state can be checked with wsrep_local_state:

http://galeracluster.com/documentation-webpages/galerastatusvariables.html#wsrep-local-state

MaxScale's Galera Monitor treats nodes in the DONOR/DESYNCED state as unavailable unless available_when_donor is configured. This ensures that MaxScale does not route queries to a node that has stale data.

https://mariadb.com/kb/en/mariadb-enterprise/mariadb-maxscale-22-galera-monitor/#available_when_donor

As far as I can tell, MariaDB Connector/J's load balancing implementation does not have a way to keep queries from being sent to desynced Galera nodes. Maybe we should add an option that would enable that kind of behavior?