@Override
  public long heapSize() {
    if (this.buf.hasArray()) {
      return ClassSize.align(FIXED_OVERHEAD + length);
    }
    return ClassSize.align(FIXED_OVERHEAD);
  }
A setter that helps to avoid object creation every time and whenever there is a need to create
new OffheapKeyOnlyKeyValue.
@param key    - the key part of the cell
@param offset - offset of the cell
@param length - length of the cell
@param rowLen - the rowlen part of the cell
hbase-common/src/main/java/org/apache/hadoop/hbase/ByteBufferKeyOnlyKeyValue.java