What was the last block processed?
        Return the number on success
        Return None on failure to read

def get_lastblock(cls, impl, working_dir):
        """
        What was the last block processed?
        Return the number on success
        Return None on failure to read
        """
        if not cls.db_exists(impl, working_dir):
            return None

        con = cls.db_open(impl, working_dir)
        query = 'SELECT MAX(block_id) FROM snapshots;'
        
        rows = cls.db_query_execute(con, query, (), verbose=False)
        ret = None
        for r in rows:
            ret = r['MAX(block_id)']

        con.close()
        return ret