Only use in-memory cache for database settings, set ttl=5 (#12166)

* Only use in-memory cache for database settings

Make necessary adjustments to monkeypatch
  as it is very vunerable to recursion
  Remove migration exception that is now redundant

Clear cache if a setting is changed

* Use dedicated middleware for setting cache stuff
  Clear cache for each request

* Add tests for in-memory cache

awx/conf/signals.py

@@ -28,6 +28,9 @@ def handle_setting_change(key, for_delete=False):
     cache_keys = {Setting.get_cache_key(k) for k in setting_keys}
     cache.delete_many(cache_keys)
 
+    # if we have changed a setting, we want to avoid mucking with the in-memory cache entirely
+    settings._awx_conf_memoizedcache.clear()
+
     # Send setting_changed signal with new value for each setting.
     for setting_key in setting_keys:
         setting_changed.send(sender=Setting, setting=setting_key, value=getattr(settings, setting_key, None), enter=not bool(for_delete))