Filestore cleanup removes referenced segments

In some situations FileStore.cleanup() may remove segments that are still referenced, subsequently causing a SNFE.

This is a regression introduced with OAK-1828.

FileStore.cleanup() relies on the ordering of the segments in the tar files: later segments only reference earlier segments. As we have seen in other places this assumption does not hold any more (e.g. OAK-3794, OAK-3793) since OAK-1828.
cleanup traverses the segments backwards maintaining a list of referenced ids. When a segment is not in that list, it is removed. However, this approach does not work with forward references as those are only seen later when the segment has been removed already.

cc alex.parvulescu, frm