Assembles the candidate block into it's finalized form for broadcast.

def build_block(self):
        """
        Assembles the candidate block into it's finalized form for broadcast.
        """
        header_bytes = self.block_header.SerializeToString()
        block = Block(header=header_bytes,
                      header_signature=self._header_signature)
        block.batches.extend(self.batches)
        return block