code: func (m *MemoryStorer) Clean() {
  	t := time.Now().UTC()
  	m.mut.Lock()
  	for id, session := range m.sessions {
  		if t.After(session.expires) {
  			delete(m.sessions, id)
  		}
  	}
  	m.mut.Unlock()
  }
docs: // Clean checks all sessions in memory to see if they are older than
  // maxAge by checking their expiry. If it finds an expired session
  // it will remove it from memory.