Map<Long, Set<Offset>> getCompactionTimestampToIndexSegmentOffsets() {
    return Collections.unmodifiableMap(compactionTimestampToIndexSegmentOffsets);
  }
Return the compaction timestamp to index segment offsets map, this should only be used for tests.
@return
ambry-store/src/main/java/com/github/ambry/store/PersistentIndex.java