Edge case lets us still use an unconfigured bridge
Our fix for #2511 (moved) made us throw away descriptors for unconfigured bridges when we read them from the disk cache. That's a good start.
But if we setconf X as our only bridge, then later setconf Y as our only bridge, we won't throw away X's descriptor so we'll still be willing to use it. Reported by anonym on #2355 (moved).
I just confirmed this bug.