@Override
  public long preFlushSeqIDEstimation() {
    if (compositeSnapshot) {
      return HConstants.NO_SEQNUM;
    }
    Segment segment = getLastSegment();
    if (segment == null) {
      return HConstants.NO_SEQNUM;
    }
    return segment.getMinSequenceId();
  }
This method is called before the flush is executed.
@return an estimation (lower bound) of the unflushed sequence id in memstore after the flush is
        executed. if memstore will be cleared returns {@code HConstants.NO_SEQNUM}.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CompactingMemStore.java