OOM needs to consider the DESTROY queued cells

Our OOM is only looking a the circuit queue cells and HS descriptors to free up memory.

We need to teach it to cleanup DESTROY cells in case cleaning up the circuits is not enough.

This isn't that trivial because while cleaning up circuits in the OOM handler, we will also send DESTROY cells for those thus allocating memory. But also not sending those will affects other relays hanging on dead circuits.

All in all, this is an interesting challenge but there might be something smart to do even if not perfect.

The idea here is to avoid an attack that takes advantage of a bug in tor that can fill up the DESTROY cell queue and our OOM just can't do anything about it.