Disabling cbt by consensus doesn't disable exploratory circuit building

At the bottom of circuit_predict_and_launch_new() we check

  /* Finally, check to see if we still need more circuits to learn
   * a good build timeout. But if we're close to our max number we
   * want, don't do another -- we want to leave a few slots open so
   * we can still build circuits preemptively as needed. */
  if (num < MAX_UNUSED_OPEN_CIRCUITS-2 &&
      get_options()->LearnCircuitBuildTimeout &&
      circuit_build_times_needs_circuits_now(&circ_times)) {
    flags = CIRCLAUNCH_NEED_CAPACITY;
    log_info(LD_CIRC,
             "Have %d clean circs need another buildtime test circ.", num);
    circuit_launch(CIRCUIT_PURPOSE_C_GENERAL, flags);
    return;
  }

Here we check get_options()->LearnCircuitBuildTimeout directly, rather than calling circuit_build_times_disabled(). That means we do these exploratory circuits even when cbt is disabled by consensus, or when we're a directory authority, or when we've failed to write cbt history to our state file lately.

We also have the same bug in circuit_expire_old_circuits_clientside() where

  if (get_options()->LearnCircuitBuildTimeout &&
      circuit_build_times_needs_circuits(&circ_times)) {
    /* Circuits should be shorter lived if we need more of them
     * for learning a good build timeout */
    cutoff.tv_sec -= IDLE_TIMEOUT_WHILE_LEARNING;
  } else {
    cutoff.tv_sec -= get_options()->CircuitIdleTimeout;
  }