Remove hole in the middle

There is additional cleanup that will come with the `DirPath` merge but at least the hole-in-the-middle is gone now.
We could but that would be inlining the `fromSnapshotIdAfter` code in the `Chord` code. I prefer to leave it close to the `FeatureStoreSnapshot` code so that it can be reused by other classes and doesn't clutter the `Chord` code too much. In general I think it is a good idea to push back those various access methods to their own layer.