code: func (l *SSHTunnelList) PickRandomTunnel() (SSHTunnelEntry, error) {
  	if len(l.entries) == 0 {
  		return SSHTunnelEntry{}, fmt.Errorf("empty tunnel list.")
  	}
  	n := mathrand.Intn(len(l.entries))
  	return l.entries[n], nil
  }
docs: /* this will make sense if we move the lock into SSHTunnelList.
  func (l *SSHTunnelList) Dial(network, addr string) (net.Conn, error) {
  	if len(l.entries) == 0 {
  		return nil, fmt.Errorf("empty tunnel list.")
  	}
  	n := mathrand.Intn(len(l.entries))
  	return l.entries[n].Tunnel.Dial(network, addr)
  }
  */
  // Returns a random tunnel, xor an error if there are none.