Time out quicker on microdesc fetch failures while we're bootstrapping
#9946 (moved) points out that "if one of our three guards is a bust, it will take 120 seconds for requests to it to time out, and even if the other 2 guards are working great, that typically isn't enough to get to 80% bootstrapped. So there will remain cases where we wait 2 minutes to bootstrap -- and maybe this patch even increases the chance of those since any of the three guards can cause them."
We should be more eager to either rotate to a different guard, or try another request in parallel, if a) we're not bootstrapped yet, b) we have directory questions we want answers to, and c) we've recently gotten good answers from our other guards.