Major Description
Today IW has try/finally around places (flush, merge) that write
per-segment files and on exception uses IFD.refresh(String) to remove
any newly created but now unreferenced files.
But, since merge exceptions are now tragic, and IFD.refresh() (not
taking a segment name) will be called, doing the same thing, I think
it's safe to remove the per-segment refresh.
This makes IW a little bit simpler ... and it means at least one fewer Directory.listAll while handling merge exceptions.