Reduce DAS queries with --onlypost

Thanks to @agbet and some network problems, we saw that when running the postprocessing step all file names were resolved (so one das query per sample, if not cached) - this brings it to one (at most, if there is an easier way to get a file for any sample that will be used). The final goal is 0, but this should already bring a nice speed improvement (and includes a small fix that was in !80 (merged) before), so it could be merged by itself.