rafthttp: update term when AddPeer

Update term when AddPeer, or the term in peer will not be updated until
the term is changed. This fixes the log flood happended when the v2.1
follower applies the snapshot from v2.0 leader:

```
rafthttp: cannot attach out of data stream server [0 / 17]
```
or
```
rafthttp: server streaming to 6e3bd23ae5f1eae0 at term 0 has been
stopped
```

rafthttp/transport.go

@@ -79,8 +79,8 @@ type transport struct {
 	serverStats  *stats.ServerStats
 	leaderStats  *stats.LeaderStats
 
+	mu      sync.RWMutex         // protect the term, remote and peer map
 	term    uint64               // the latest term that has been observed
-	mu      sync.RWMutex         // protect the remote and peer map
 	remotes map[types.ID]*remote // remotes map that helps newly joined member to catch up
 	peers   map[types.ID]Peer    // peers map
 	errorc  chan error
@@ -116,6 +116,8 @@ func (t *transport) Get(id types.ID) Peer {
 }
 
 func (t *transport) maybeUpdatePeersTerm(term uint64) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
 	if t.term >= term {
 		return
 	}
@@ -192,7 +194,9 @@ func (t *transport) AddPeer(id types.ID, us []string) {
 		plog.Panicf("newURLs %+v should never fail: %+v", us, err)
 	}
 	fs := t.leaderStats.Follower(id.String())
-	t.peers[id] = startPeer(t.roundTripper, urls, t.id, id, t.clusterID, t.raft, fs, t.errorc)
+	p := startPeer(t.roundTripper, urls, t.id, id, t.clusterID, t.raft, fs, t.errorc)
+	p.setTerm(t.term)
+	t.peers[id] = p
 }
 
 func (t *transport) RemovePeer(id types.ID) {

rafthttp/transport_test.go

@@ -67,19 +67,21 @@ func TestTransportSend(t *testing.T) {
 
 func TestTransportAdd(t *testing.T) {
 	ls := stats.NewLeaderStats("")
+	term := uint64(10)
 	tr := &transport{
 		roundTripper: &roundTripperRecorder{},
 		leaderStats:  ls,
+		term:         term,
 		peers:        make(map[types.ID]Peer),
 	}
 	tr.AddPeer(1, []string{"http://localhost:2380"})
-	defer tr.Stop()
 
 	if _, ok := ls.Followers["1"]; !ok {
 		t.Errorf("FollowerStats[1] is nil, want exists")
 	}
 	s, ok := tr.peers[types.ID(1)]
 	if !ok {
+		tr.Stop()
 		t.Fatalf("senders[1] is nil, want exists")
 	}
 
@@ -89,6 +91,12 @@ func TestTransportAdd(t *testing.T) {
 	if s != ns {
 		t.Errorf("sender = %v, want %v", ns, s)
 	}
+
+	tr.Stop()
+
+	if g := s.(*peer).msgAppReader.msgAppTerm; g != term {
+		t.Errorf("peer.term = %d, want %d", g, term)
+	}
 }
 
 func TestTransportRemove(t *testing.T) {