Revise /hssvc-ipt-algorithms.md based on updated client behavior assumptions

Also see #966.

@Diziet asked me to open this ticket to track the work of revising doc/dev/notes/hssvc-ipt-algorithms.md to account for the fact that clients will sometimes retry downloading onion service descriptors before they expire. This means, for example, that we do not need to remember every non-expired descriptor we've ever uploaded.