Circuit Build Timeout should ignore abandon circuits, lower min timeout
While working on #40157, @asn and I discovered that the Circuit Build Timeout learned by CBT was too high. It is counting failed circuits and circuits that did not complete as if they were the longest build-time ("right-censored" in the Pareto estimator). This caused the resulting Pareto curve to be a poor fit. These circuits weren't actually that slow; their relays were just down or unresponsive. We should not count these circuits in our build time stats. Doing so in onionperf testing makes the Pareto curve fit much tighter.
Additionally, there is a #define CBT_DEFAULT_TIMEOUT_MIN_VALUE that makes the minimum timeout value 1.5 seconds. This is way too high -- timeouts on the network range from 80ms to 500ms now. This minimum should be like 25 or 50ms now.
This should be backported to stable clients for a couple reasons -- we want to eventually tune the network-wide circuit build timeout quantile for Sponsor61 experiments, but we can't do that until all clients compute the same circuit build timeout. Additionally, it is abysmal UX to wait for 1.5 seconds on circuits that will never ever build.