@Override
  public void positionToFirst() {

    // Reset this before calling positionToFirst.
    bufferOffset = 0;

    super.positionToFirst();

    // This is constant for whole series.
    serializedBytes = output.getData();
  }
Batch compute the hash codes for all the serialized keys.

NOTE: MAJOR MAJOR ASSUMPTION:
    We use Murmur3.hash32(seed=0) across the board for the ReduceSink UNIFORM distribution.
    Previous use of HashCodeUtil is deprecated.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/keyseries/VectorKeySeriesSerializedImpl.java