Adds more health check info to the galera cluster status plugin.

Adds some more information needed to fully monitor a galera cluster:

* flow_control_paused: if it's > 0 then there's a problem and replication's been stopped.
* local_recv_queue_avg: the higher the number the farther it is behind the other members of the cluster.
* local_send_queue_avg: the higher the number, the longer it takes for this node to send its changes to the other members of the cluster, a high number probably means network performance issues.
* cluster_size: this is good to know and alert on - if it's less than you expect, you have a problem and need to fix it pretty darn quick.

Thanks!