etcdserver: support update cluster version through raft

1. Persist the cluster version change through raft. When the member is restarted, it can recover
the previous known decided cluster version.

2. When there is a new leader, it is forced to do a version checking immediately. This helps to
update the first cluster version fast.

etcdserver/server_test.go

@@ -1041,6 +1041,45 @@ func TestPublishRetry(t *testing.T) {
 	}
 }
 
+func TestUpdateVersion(t *testing.T) {
+	n := &nodeRecorder{}
+	ch := make(chan interface{}, 1)
+	// simulate that request has gone through consensus
+	ch <- Response{}
+	w := &waitWithResponse{ch: ch}
+	srv := &EtcdServer{
+		id:         1,
+		r:          raftNode{Node: n},
+		attributes: Attributes{Name: "node1", ClientURLs: []string{"http://node1.com"}},
+		Cluster:    &Cluster{},
+		w:          w,
+		reqIDGen:   idutil.NewGenerator(0, time.Time{}),
+	}
+	srv.updateClusterVersion("2.0.0")
+
+	action := n.Action()
+	if len(action) != 1 {
+		t.Fatalf("len(action) = %d, want 1", len(action))
+	}
+	if action[0].Name != "Propose" {
+		t.Fatalf("action = %s, want Propose", action[0].Name)
+	}
+	data := action[0].Params[0].([]byte)
+	var r pb.Request
+	if err := r.Unmarshal(data); err != nil {
+		t.Fatalf("unmarshal request error: %v", err)
+	}
+	if r.Method != "PUT" {
+		t.Errorf("method = %s, want PUT", r.Method)
+	}
+	if wpath := path.Join(StoreClusterPrefix, "version"); r.Path != wpath {
+		t.Errorf("path = %s, want %s", r.Path, wpath)
+	}
+	if r.Val != "2.0.0" {
+		t.Errorf("val = %s, want %s", r.Val, "2.0.0")
+	}
+}
+
 func TestStopNotify(t *testing.T) {
 	s := &EtcdServer{
 		stop: make(chan struct{}),