Avoid memory allocation in OOM handler

Right now, the OOM handler allocates an array of size(n_circs * sizeof(void*)) to figure out which circuits to kill. That's probably not a good idea. Instead, it would be better to select the select circuits from the circuitlist.

Options here include:

• Turn global_circuitlist into an array (smartlist) structure.
• Retain the linked-list structure, but do one of these:
  • Use a merge sort to sort the circuitlist in place.
  • Do an O(N^2) algorithm to walk the circuitlist to find the worst circuit, delete that, and then do it over and over until we have killed enough circuits.
  • Do an O(Nk) algorithm to walk the circuitlist to find the k worst circuits; kill them; repeat until we have killed enough circuits. This still turns out to O(N^2)