@Override
  public byte[] toByteArray(int offset, int length) {
    // No usage of this API in code. Just making it as an Unsupported operation as of now
    throw new UnsupportedOperationException();
  }
Release the resources it uses (The ByteBuffers) which are obtained from pool. Call this only
when all the data is fully used. And it must be called at the end of usage else we will leak
ByteBuffers from pool.
hbase-common/src/main/java/org/apache/hadoop/hbase/io/ByteBufferListOutputStream.java