etcdserver: enable "CheckQuorum" when starting with "ForceNewCluster"

[gyuho]

We enable "raft.Config.CheckQuorum" by default in other
Raft initial starts. So should start with "ForceNewCluster".

If new members are added to standalone node with CheckQuorum == false and the standalone node is the leader, this may reduce cluster availabilities, since the leader won't check quorum activities.

For current etcd, it won't help a lot, but once we enable Pre-Vote, this will be useful. For instance, pb.MsgAppResp from isolated candidate may be dropped or not even be triggered with Pre-Vote, and then pb.Msg[Pre]Vote arrives with higher term. And if the check quorum is false, the vote request would force current leader to step down even if the leader lease is still valid. If check quorum is true, the vote request will be ignored when its leader lease is still valid, thus less disruptive.

Reproducible (will write a better test case after pre-vote feature):

func TestForceNewClusterCheckQuorum(t *testing.T) {
	defer testutil.AfterTest(t)

	clus := NewClusterV3(t, &ClusterConfig{Size: 3})
	defer clus.Terminate(t)

	clus.WaitLeader(t)

	_, err := clus.Client(0).Put(context.TODO(), "foo", "bar")
	if err != nil {
		t.Fatal(err)
	}

	clus.Members[0].Stop(t)
	clus.Members[1].Terminate(t)
	clus.Members[2].Terminate(t)
	clus.Members = clus.Members[:1]
	clus.Members[0].ForceNewCluster = true
	clus.Members[0].ElectionTicks *= 2 // greater leader lease

	if err = clus.Members[0].Restart(t); err != nil {
		t.Fatalf("unexpected ForceRestart error: %v", err)
	}

	resp, err := clus.Client(0).MemberList(context.Background())
	if err != nil {
		t.Fatal(err)
	}
	if len(resp.Members) != 1 {
		t.Fatalf("expected len(resp.Members) == 1, got %d", len(resp.Members))
	}

	clus.AddMember(t)
	clus.WaitLeader(t)

	clus.AddMember(t)
	lead := clus.WaitLeader(t)
	if lead != 0 {
		t.Skip()
	}

	// m[0] is leader, started with raft.Config.CheckQuorum == false
	// m[1] started with raft.Config.CheckQuorum == true
	// m[2] started with raft.Config.CheckQuorum == true

	// drop incoming messages to m[2]
	// m[2] will vote request to m[0]
	clus.Members[0].s.PauseSending()

	// prevent m[1] from voting for m[0]
	// and prevent m[2] from becoming leader
	clus.Members[1].s.PauseSending()

	// make m[2] candidate
	time.Sleep(clus.Members[2].ElectionTimeout() + 100*time.Millisecond)

	clus.Members[0].s.ResumeSending()

	// force leader m[0] to step down by sending
	//   1. MsgAppResp with higher term
	//   2. MsgVote with higher term
	time.Sleep(clus.Members[2].ElectionTimeout())

	// m[0] CheckQuorum == true, m[0] leader lease is still valid,
	// and MsgVote arrives firt, then m[0] would ignore MsgVote from m[2]
	// and m[0] may win the next election over m[2]
	//
	// m[0] CheckQuorum == false, whether m[0] leader lease is still
	// valid or not, MsgVote from m[2] would force m[0] to step down
	// right away
}

[gyuho]

@xiang90 Think we forgot to set this from #4779?