Validate flag-thresholds in consensus-health checker
As of yesterday, votes may contain
flag-thresholds lines like this one from moria1 (line breaks only added here):
flag-thresholds stable-uptime=693369 stable-mtbf=153249 fast-speed=40960 guard-wfu=94.669% guard-tk=691200 guard-bw-inc-exits=174080 guard-bw-exc-exits=184320 enough-mtbf=1
The consensus-health checker can look at these values and warn if one of them gets too high or too low. What values should it consider normal, and when should it begin warning?