api.go
func (r *Raft) Leader() ServerAddress {
	r.leaderLock.RLock()
	leader := r.leader
	r.leaderLock.RUnlock()
	return leader
}
func (r *Raft) Leader() ServerAddress {
	r.leaderLock.RLock()
	leader := r.leader
	r.leaderLock.RUnlock()
	return leader
}
// Leader is used to return the current leader of the cluster.
// It may return empty string if there is no current leader
// or the leader is unknown.