Offset getActiveIndexSegmentOffset() {
    Offset logEndOffset = log.getEndOffset();
    Map.Entry<Offset, IndexSegment> lastEntry = validIndexSegments.lastEntry();
    // Fetch the log end offset before last key in the map
    if (lastEntry != null) {
      return lastEntry.getKey();
    } else {
      // No index segment in the map, then this log is empty. End offset is the start offset.
      logger.trace("Index {}: returning log end offset as the active index segment offset: {}", dataDir, logEndOffset);
      return logEndOffset;
    }
  }
Return the start offset of the active index segment. If there is no index segment at all, just return the beginning
offset of the entire log.
@return
ambry-store/src/main/java/com/github/ambry/store/PersistentIndex.java