code: func (cluster *mongoCluster) Release() {
  	cluster.Lock()
  	if cluster.references == 0 {
  		panic("cluster.Release() with references == 0")
  	}
  	cluster.references--
  	debugf("Cluster %p released (refs=%d)", cluster, cluster.references)
  	if cluster.references == 0 {
  		for _, server := range cluster.servers.Slice() {
  			server.Close()
  		}
  		// Wake up the sync loop so it can die.
  		cluster.syncServers()
  		stats.cluster(-1)
  	}
  	cluster.Unlock()
  }
docs: // Release decreases the reference count for the cluster. Once
  // it reaches zero, all servers will be closed.