@Override
  public boolean isStopped() {
    return this.stopped;
  }
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