NavigableMap<Offset, Offset> getBeforeAndAfterIndexSegmentOffsets() {
    return Collections.unmodifiableNavigableMap(beforeAndAfterIndexSegmentOffsets);
  }
@return The index segment offset map. Each key and value is before and after pair passed to {@link #addBeforeAndAfterIndexSegmentOffsetPair}.
ambry-store/src/main/java/com/github/ambry/store/CompactionLog.java