Initialize list of transaction.
        Args:
            req_begin_block: block object which contains block header
            and block hash.

def begin_block(self, req_begin_block):
        """Initialize list of transaction.
        Args:
            req_begin_block: block object which contains block header
            and block hash.
        """
        self.abort_if_abci_chain_is_not_synced()

        chain_shift = 0 if self.chain is None else self.chain['height']
        logger.debug('BEGIN BLOCK, height:%s, num_txs:%s',
                     req_begin_block.header.height + chain_shift,
                     req_begin_block.header.num_txs)

        self.block_txn_ids = []
        self.block_transactions = []
        return ResponseBeginBlock()