Get cached consensus info.
        Returns {...} on success
        Return None if it is stale

def get_cached_consensus_info(self):
        """
        Get cached consensus info.
        Returns {...} on success
        Return None if it is stale
        """
        cached_consensus_info = self.cache.get('consensus_info', None)
        if cached_consensus_info is None:
            # not cached
            return None

        now = time.time()
        if cached_consensus_info['time'] + AVERAGE_SECONDS_PER_BLOCK < now:
            # stale
            return None

        return cached_consensus_info['info']