Some fixes to NodeContainingKey

I removed an extra FdbTuple.Pack and switched from UnpackSingle to UnpackFirst. I fixed an asymptomatic bug that is also present in the python directory layer, where the subspace returned has the wrong prefix (which currently is never used).

I noticed that UnpackSingle's documentation says that it will throw an error if the tuple has more than one element, but it currently doesn't. I didn't make any changes to this method.