Prepare relay descriptor downloader for consensuses published at :30 of the hour
Consensuses have always been published once per hour, so with valid-after time :00 in the past. This might change in the future, as has been discussed in the context of consensus diffs, but which might also happen independent of those.
However, CollecTor is not ready for such a change, because
RelayDescriptorDownloader is always initialized with :00 of the hour. This would prevent us from accepting a consensus published at :30. Of course, when we make such a change we should also accept other valid-after times that :00 and :30.
Testing a fix might require setting up a testing network using Chutney and archiving network data produced by that. That is, we could also create a testing environment and write unit tests for this, which we should do anyway. But that shouldn't block us here, unless the fix is more complex than I currently anticipate.