Long getLogSegmentDeletedBucketKey(long referenceTimeInMs) {
    return logSegmentDeltaBucketsForDeleted.higherKey(referenceTimeInMs);
  }
Given a reference time, return the key of the appropriate log segment deleted bucket whose end time is strictly
greater than the reference time.
@param referenceTimeInMs the reference time or operation time of an event.
@return the appropriate bucket key (bucket end time) to indicate which bucket will an event with
the given reference time as operation time belong to.
ambry-store/src/main/java/com/github/ambry/store/ScanResults.java