Rewrite the sasha function for retrieving archived WUID

I create this Jira based on https://github.com/hpcc-systems/HPCC-Platform/pull/11830#pullrequestreview-165701122: I think this okay, given the other rough checks done here, but really the whole of this func. could do with rewriting to parse the expected format a little more cleanly.

But also, it would be much better to stop these files getting as far as being use to call readWUIDFromFileName(). They are being returned from a IDirectoryIterator which uses a wildcard, which is causing a lot of unintended files to be picked up. It would be to ensure they were not returned from the iterator, by using a better wild card if possible.