Don't weight by bandwidth when selecting among bridges

In choose_random_entry_impl() we have:

choose_and_finish:
  if (entry_list_is_constrained(options)) {
    /* We need to weight by bandwidth, because our bridges or entryguards
     * were not already selected proportional to their bandwidth. */
    node = node_sl_choose_by_bandwidth(live_entry_guards, WEIGHT_FOR_GUARD);

This means that bridges are also selected proportional to their bandwidth. However, since there is no bandwidth authorities for bridges their bandwidth is self-reported and potentially a lie. For this reason, it's probably not a good idea to use those values during path selection, since an evil bridge can try to dominate the guard probability.

Fortunately, we also have bridge_get_advertised_bandwidth_bounded() which bounds bridges bandwidth between 20kB/s and 100kB/s. So the danger can't be that great.

Still, it might be a better idea to pick amongst bridges in a uniform random way.