This doesn't seem to fix the case where an exception bubbles up to the top of
the event loop. It also doesn't seem to fix the case when a KeyboardInterrupt is
thrown, since that never makes its way into the event loop in the first place.
Both of these cases lead to the event loop stopping, which means that the tmp
file cleanup doesn't get executed even though it's inside a "with" or "finally".
PFERD now not only updates file metadata when a file is successfully added or
changed, but also when a file is downloaded and then detected to be unchanged.
This could occur for example if a remote file's modification time was bumped,
possibly because somebody touched the file without changing it.
