Directory Layer: fixed bugs in MoveTo and Remove that preventing renaming or deleting a root partition

- partition.MoveToAsync("new_name") should rename a partition using its parent DirectoryLayer, same for TryMoveToAsync()
- partition.RemoveAsync() should remove a partition from its parent DirectoryLayer, same for TryRemoveAsync()
- added unit tests to verify this
- added missing version of IFdbDirectory.ListAsync(IFdbReadOnlyTransaction) that does not take a path