protected void releaseIfNotCurBlock(HFileBlock block) {
      if (curBlock != block) {
        block.release();
      }
    }
The curBlock will be released by shipping or close method, so only need to consider releasing
the block, which was read from HFile before and not referenced by curBlock.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderImpl.java