@Override
    protected void cleanup(Context context) throws IOException, InterruptedException {
      hasher.finishBatch();
      context.write(hasher.getBatchStartKey(), hasher.getBatchHash());
    }
Get the current hash
@return the current hash or null if there is no current hash
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/HashTable.java