code: func NewMemoryBackend() *MemoryBackend {
  	return &MemoryBackend{
  		SessionQueueSize:  100,
  		KillTimeout:       5 * time.Second,
  		activeClients:     make(map[string]*Client),
  		storedSessions:    make(map[string]*memorySession),
  		temporarySessions: make(map[*Client]*memorySession),
  		retainedMessages:  topic.NewTree(),
  	}
  }
docs: // NewMemoryBackend returns a new MemoryBackend.