@return Snapshottable directories that are owned by the current user,
   *         represented as an array of {@link SnapshottableDirectoryStatus}. If
   *         {@code userName} is null, return all the snapshottable dirs.
@return Snapshottable directories that are owned by the current user,
   *         represented as an array of {@link SnapshottableDirectoryStatus}. If
   *         {@code userName} is null, return all the snapshottable dirs. | SnapshotManager.java