Take descriptor upload overlap into account when estimating version 3 onion address counts
Based on this tor metrics paper: https://research.torproject.org/techreports/extrapolating-hidserv-stats-2015-01-31.pdf
We ignore descriptor upload overlap periods (they're not even mentioned in the paper).
During an overlap period, descriptors are published to twice as many HSDirs (v2 & v3). If we ignore this, we will double-count:
- v2: uploads and unique onion addresses and descriptor ids for 1 hour per day,
- v3: uploads and unique descriptor ids for 12 hours per day.
I'm not sure if assuming that descriptors are seen by 2 sets of HSDirs per day covers this, because in v2 they are actually seen by 3 sets of HSDirs with probability 1/24, when the service address starts with 00-0B (assuming stats are collected from 00:00 UTC and relay clocks are accurate).
And in v3, for half the day (00:00 + typical client consensus download delay of 1-2 hours) they are seen by 2 HSDirs, and half the day they are seen by 1 HSDir. Not that we measure v3 yet.