While working through
LUCENE-710 I hit this bug: on Windows
only, when SegmentReader.setNorm is called, but separate norms
(_X_N.sY) had already been previously saved, then, on closing the
reader, we will write the next gen separate norm file correctly
(_X_N+1.sY) but fail to delete the current one.
It's quite minor because the next writer to touch the index will
remove the stale file.
This is because the Norm class still holds the IndexInput open when
the reader commits.