Expose online site names

In Infinispan we use the sites up/down events to compute an "xsite view" (logging purpose only).

However, the events are not reliable, i.e. it depends on how the nodes start. Usually, only the coordinator ends up with the correct set of events (and the correct "xsite view") and the other site masters (max_site_master >1) only have a partial or no events at all.

It would be nice if RELAY2 exposes relayer.getSiteNames() and returns null if the node isn't the site master.