code: func blockIndexKey(blockHash *chainhash.Hash, blockHeight uint32) []byte {
  	indexKey := make([]byte, chainhash.HashSize+4)
  	binary.BigEndian.PutUint32(indexKey[0:4], blockHeight)
  	copy(indexKey[4:chainhash.HashSize+4], blockHash[:])
  	return indexKey
  }
docs: // blockIndexKey generates the binary key for an entry in the block index
  // bucket. The key is composed of the block height encoded as a big-endian
  // 32-bit unsigned int followed by the 32 byte block hash.