@Override
  public void stop(String why) {
    if (!this.stopped) {
      this.stopped = true;
      LOG.info("Stopping BackupLogCleaner");
    }
  }
Calculates the timestamp boundary up to which all backup roots have already included the WAL.
I.e. WALs with a lower (= older) or equal timestamp are no longer needed for future incremental
backups.
hbase-backup/src/main/java/org/apache/hadoop/hbase/backup/master/BackupLogCleaner.java