Default node ID should be unique

In a cluster, each Openfire node has an identifier (as defined by org.jivesoftware.openfire.XMPPServer#getNodeID). When not running in a cluster, a default value is used.

When joining a cluster, there's a risk that the default value remains in use (at least for a short while).

Currently, the default value is equal on each server, which means that in a cluster, each node will assume that the associated reference was added by the local cluster node.

To prevent issues, the default value used on servers should be unique.