public Map<TableName, Map<String, Long>> readLogTimestampMap() throws IOException {
    return systemTable.readLogTimestampMap(backupInfo.getBackupRootDir());
  }
Read the timestamp for each region server log after the last successful backup. Each table has
its own set of the timestamps.
@return the timestamp for each region server. key: tableName value:
        RegionServer,PreviousTimeStamp
@throws IOException exception
hbase-backup/src/main/java/org/apache/hadoop/hbase/backup/impl/BackupManager.java