@Override
  public long getHighWatermark() {
    return highWatermark;
  }
txnlist is represented in ranges like this: 10-20,14,16-50
if the range is smaller then 5 it will not get merged, to avoid unnecessary overhead
@param txnListString string to parse from
@return txn list
storage-api/src/java/org/apache/hadoop/hive/common/ValidReadTxnList.java