hs: Make HSDir produce HS statistics for prop224

You can find in hs_cache.c this line which indicates the callsite where we should gather the "uniqueness of .onion".

/* XXX: Update HS statistics. We should have specific stats for v3. */

Now here is the tricky part. HSDir can't learn a .onion address anymore with prop224 so counting the unique blinded key we see could do the trick but might give us a different value from what we have now. Blinded keys will change every ~24h which is the timeframe where every relay currently reset the HS stats so it could be fine as a representation of a unique .onion.

This will require a dir-spec change as we'll have to add something like this because we can't use the v2 stats (hidserv-dir-onions-seen):

    "hidserv-dir-blindedkey-seen" SP num SP key=val SP key=val ... NL

Last thing, @cypherpunks, we know that you don't like that so when commenting here, please tell us why you don't want it with some useful argumentation. Thanks ;).