func (r *Raft) RegisterObserver(or *Observer) {
	r.observersLock.Lock()
	defer r.observersLock.Unlock()
	r.observers[or.id] = or
}
func (r *Raft) RegisterObserver(or *Observer) {
	r.observersLock.Lock()
	defer r.observersLock.Unlock()
	r.observers[or.id] = or
}
// RegisterObserver registers a new observer.