observer.go
func (r *Raft) DeregisterObserver(or *Observer) {
	r.observersLock.Lock()
	defer r.observersLock.Unlock()
	delete(r.observers, or.id)
}
func (r *Raft) DeregisterObserver(or *Observer) {
	r.observersLock.Lock()
	defer r.observersLock.Unlock()
	delete(r.observers, or.id)
}
// DeregisterObserver deregisters an observer.