Write a hidden service hsdir health measurer

A given hidden service maps to six hsdirs (hidden service directories) at any given time, in terms of where it will publish its hsdesc (hidden service descriptor).

But due to churn in relays, and different perspectives on the network (since clients and the hidden service likely don't have the very latest consensus), there's a chance that the client will choose an hsdir that doesn't have the hsdesc.

It would be neat to look at the last few consensuses, and compute the union of all the hsdirs that might be in use, and then see which ones actually have the hsdesc.

That answer will give us a handle on whether things are going as we expect, and also give us a sense of whether the number "six" is overkill or underkill or what.

An alternate, perhaps parallel, approach would be to look at the historical metrics data involving relay churn (easy to see in the consensus) and relay restarts (where uptime resets) and calculate what the answer ought to be. And if our prediction is much different from the reality, why?