prop224: Update spec wrt overlap behavior of HSes
Teor asked a few questions wrt HS overlap logic and we started digging into the implementation to see if it conforms to the spec: https://lists.torproject.org/pipermail/tor-dev/2017-June/012336.html
It seems like the code does not do the 36 hour expiration logic, and instead it generates HSDir indices for all nodes upon receiving a new consensus.
We should verify that the logic in the code makes sense and has no problems, and then update the spec to specify the right behavior.