code: public void close(C1NChunk c, int cidx, Futures fs) {
      assert _len==-1;            // Not closed
      c._vec = this;              // Attach chunk to this vec.
      DKV.put(chunkKey(cidx),c,fs); // Write updated chunk back into K/V
      _len = ((_nchunks-1L)<<H2O.LOG_CHK)+c._len;
    }
docs: Close, and possible replace the prior chunk with a new, larger Chunk