Fixed concurrency error.

git-svn-id: http://svn.igniterealtime.org/svn/repos/openfire/trunk@8672 b35dd754-fafc-0310-a699-88a17e54d16e

src/java/org/jivesoftware/openfire/spi/RoutingTableImpl.java

@@ -539,7 +539,7 @@ public class RoutingTableImpl extends BasicModule implements RoutingTable, Clust
     }
 
     public Collection<String> getServerHostnames() {
-        return serversCache.keySet();
+        return new ArrayList<String>(serversCache.keySet());
     }
 
     public boolean hasClientRoute(JID jid) {