Improve RELAY/RELAY2 JGroups protocol selection

When using x-site replication, there is the choice of using RELAY or RELAY2 JGroups protocols. The existence of choice is not obvious and not well documented.

As an example, if using SYNC x-site replication and RELAY2, it's easy to end up in this situation: if I define site A to have site B as backup, and site B is simply not up yet, why should a put in site A fail? In a normal cluster, if I have node X and node Y is not up yet, a put on node X simply does not fail. To make this work, you need to configure RELAY instead of RELAY2.

The RELAY2 doc is very explicit that it doesn't want to expose a virtual view, the sites should be configured statically, and sites that are not up are considered unreachable (https://github.com/belaban/JGroups/blob/master/doc/design/RELAY2.txt)