@Override
  public int compareTo(IndexValue o) {
    return offset.compareTo(o.offset);
  }
{@inheritDoc}
<p/>
Compares by {@link Offset}.
@param o the {@link IndexValue} to compare to.
@return a negative integer, zero, or a positive integer as the offset of this {@link IndexValue} is less than,
equal to, or greater than the offset of {@code o}.
ambry-store/src/main/java/com/github/ambry/store/IndexValue.java