State API shouldn't be used for caching

[MPParsley]

In GroupTypeManager::refreshGroupRelationMap() state API is used to cache the map:

    // Cache the map.
    $this->state->set(self::GROUP_RELATION_MAP_CACHE_KEY, $this->groupRelationMap);

We shouldn't use state for caching. It can break things when another entity type has an og_reference field (e.g. og_menu).

A simple workaround is to clear the state manually:

drush state:delete og.group_manager.group_relation_map

[pfrenssen]

Good find! We should use the Cache API for this.