@Override
  public HardDeleteInfo next() {
    if (!hasNext()) {
      throw new NoSuchElementException();
    }
    return getHardDeleteInfo(readSetIndex++);
  }
This class takes a read set for blobs that are to be hard deleted and provides corresponding
replacement messages, that can then be written back by the caller to hard delete those blobs.
ambry-messageformat/src/main/java/com/github/ambry/messageformat/BlobStoreHardDelete.java