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?