2 - Improve the circuit queue out-of-memory handler. Previously, when
3 we ran low on memory, we'd close whichever circuits had the most
4 queued cells. Now, we close those that have the *oldest* queued
5 cells, on the theory that those are most responsible for us
6 running low on memory. Based on analysis from a forthcoming paper
7 by Jansen, Tschorsch, Johnson, and Scheuermann. Fixes bug 9093.