Add toString method to probe client

src/refit/application/probe/REFITProbeClient.java

@@ -23,6 +23,11 @@ public class REFITProbeClient extends REFITClientThread {
 	private boolean firstRun = true;
 	private List<REFITProbeData> data = new ArrayList<>();
 
+	@Override
+	public String toString() {
+		return "PRBC-" + (service.getNodeID() - REFITConfig.TOTAL_NR_OF_REPLICAS);
+	}
+
 	@Override
 	protected void work() throws Exception {
 		int clientGroup = REFITReplicaGroups.getClientGroup(service.getNodeID());