@Override
  public ExtendedCell getFirstKeyCellInBlock(ByteBuff block) {
    block.mark();
    int keyLength = block.getInt();
    block.getInt();
    ByteBuffer key = block.asSubByteBuffer(keyLength).duplicate();
    block.reset();
    return createFirstKeyCell(key, keyLength);
  }
Store cells following every row's start offset, so we can binary search to a row's cells. Format:
flat cells integer: number of rows integer: row0's offset integer: row1's offset .... integer:
dataSize
hbase-common/src/main/java/org/apache/hadoop/hbase/io/encoding/RowIndexCodecV1.java