Maybe we want three preemptive internal circs for hidden services?

  /* Fourth, see if we need any more hidden service (client) circuits. */
  if (rep_hist_get_predicted_internal(now, &hidserv_needs_uptime,
                                      &hidserv_needs_capacity) &&
      ((num_uptime_internal<2 && hidserv_needs_uptime) ||
        num_internal<2)) {

Right now we build 2 preemptive internal circuits if we've accessed a hidden service lately.

But when we click on a new .onion address, we use three circuits -- one to do the hsdesc fetch, one for the rend point, and one for the intro point.

You can see it in this walkthrough: https://trac.torproject.org/projects/tor/wiki/doc/TorControlPortWalkthrough-HS where we cannibalize the hsdesc fetch circ, cannibalize the rend point, and then have to build the intro point from scratch.

We could raise the above '2' to '3'. I'm not sure if it's worthwhile in practice though. Sure would be nice to have some way of answering that question.