DNS test predicted circuits

Arthur and Dennis have detected that some long tail of exits/relays are really bad for unknown reasons. Sometimes this is just DNS config, but there are other things beyond that.

We could DNS test our circuits and toss out the slowest 5%, via a mechanism similar to CBT math. We don't really need to toss out a lot of circuits via this mechanism, as something like 1% or less of circuits have this problem. But the ones that do have it delay page load by 6 seconds or more.