circmgr: back off on preemptive circuits if they fail consistently
Rather than running preemptive circuit construction every 10 seconds, we change it to back off when it is "failing". (We define "failing" as creating no new circuits, and as giving at least one error.)
This change means that we'll have one less reason to hammer the network when our connectivity is failed for some reason.
Closes #437 (closed). Part of #329 (closed).