private long getTimestamp(String backupId) {
        return Long.parseLong(Iterators.get(Splitter.on('_').split(backupId).iterator(), 1));
      }
Given the backup root dir and the backup id, return the log file location for an incremental
backup.
@param backupRootDir backup root directory
@param backupId      backup id
@return logBackupDir: ".../user/biadmin/backup1/WALs/backup_1396650096738"
hbase-backup/src/main/java/org/apache/hadoop/hbase/backup/util/BackupUtils.java