Also: there is a subset of hard failures for which we *know* no server-side orphan was created, and that the obsolete documents we're tracking don't need to be changed (because there's no way they contributed to the failure) -- DNS lookups, for example.

I think we can save a lot of overhead here if we spot those transport errors and short-circuit this tracker work, as well as *removing the document ID we just tracked*. After all, if we weren't able to open a socket to the Bagheera server, we know for sure the document wasn't uploaded, right?