Prop#329 OOM: Handle freeing conflux queues on OOM

Prop#329 OOM: Handle freeing conflux queues on OOM

We use the oldest-circ-first method here, since that seems good for conflux:
queues could briefly spike, but the bad case is if they are maliciously
bloated to stick around for a long time.

The tradeoff here is that it is possible to kill old circuits on a relay
quickly, but that has always been the case with this algorithm choice.

Signed-off-by: David Goulet <dgoulet@torproject.org>