raft.go
func (r *Raft) setLeader(leader ServerAddress) {
	r.leaderLock.Lock()
	oldLeader := r.leader
	r.leader = leader
	r.leaderLock.Unlock()
	if oldLeader != leader {
		r.observe(LeaderObservation{leader: leader})
	}
}
func (r *Raft) setLeader(leader ServerAddress) {
	r.leaderLock.Lock()
	oldLeader := r.leader
	r.leader = leader
	r.leaderLock.Unlock()
	if oldLeader != leader {
		r.observe(LeaderObservation{leader: leader})
	}
}
// setLeader is used to modify the current leader of the cluster