Map<Offset, Offset> getBeforeAndAfterCompactionIndexSegmentOffsets() {
    return Collections.unmodifiableMap(beforeAndAfterCompactionIndexSegmentOffsets);
  }
Return the before and after compaction index segment offsets map, this should only be used for tests.
@return
ambry-store/src/main/java/com/github/ambry/store/PersistentIndex.java