Get the number of Blockstack transactions that occured at the given block.
        Returns {'count': ..} on success
        Returns {'error': ...} on error

def rpc_get_num_blockstack_ops_at(self, block_id, **con_info):
        """
        Get the number of Blockstack transactions that occured at the given block.
        Returns {'count': ..} on success
        Returns {'error': ...} on error
        """
        if not check_block(block_id):
            return {'error': 'Invalid block height', 'http_status': 400}

        db = get_db_state(self.working_dir)
        count = db.get_num_blockstack_ops_at( block_id )
        db.close()

        log.debug("{} name operations at {}".format(count, block_id))
        return self.success_response({'count': count})