public String readBackupStartCode() throws IOException {
    return systemTable.readBackupStartCode(backupInfo.getBackupRootDir());
  }
Read the last backup start code (timestamp) of last successful backup. Will return null if
there is no startcode stored in backup system table or the value is of length 0. These two
cases indicate there is no successful backup completed so far.
@return the timestamp of a last successful backup
@throws IOException exception
hbase-backup/src/main/java/org/apache/hadoop/hbase/backup/impl/BackupManager.java