public long getOffset() {
    if (offset < 0) {
      throw new IllegalStateException("HFile block offset not initialized properly");
    }
    return offset;
  }
Cannot be {@link #UNSET}. Must be a legitimate value. Used re-making the {@link BlockCacheKey}
when block is returned to the cache.
@return the offset of this block in the file it was read from
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlock.java