void reset() {
      // We don't have to keep ref to heap block
      if (this.curBlock != null && this.curBlock.isSharedMem()) {
        this.prevBlocks.add(this.curBlock);
      }
      this.curBlock = null;
    }
The next indexed key is to keep track of the indexed key of the next data block. If the
nextIndexedKey is HConstants.NO_NEXT_INDEXED_KEY, it means that the current data block is the
last data block. If the nextIndexedKey is null, it means the nextIndexedKey has not been
loaded yet.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderImpl.java