IF we decide to upload hsdescs for the _next_ time period, make our OPE support that

See discussion at #1166 (closed); right now our OPE revision-counter algorithm assumes that you won't ever try to upload for a time period that starts in your future, but we think that our code might want to do that.

If we decide that our code does want to do that, we'll need to update our OPE approach to support it.