It appears as though performing a lot of requests is the most costly
aspect of listing comment counts per topic; this change aims to address
that by reading all the counts in a single request.
On my computer, this changes loading times from an absolute 100-200 ms to
50-100 ms most of the time. The request impact is not easy to measure as a
couple ms is small enough to fall in the range of disk access and other
semi-random events.