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/settings/defaults.py

@@ -955,6 +955,7 @@ RECEPTOR_RELEASE_WORK = True
 
 MIDDLEWARE = [
     'django_guid.middleware.guid_middleware',
+    'awx.main.middleware.SettingsCacheMiddleware',
     'awx.main.middleware.TimingMiddleware',
     'django.contrib.sessions.middleware.SessionMiddleware',
     'awx.main.middleware.MigrationRanCheckMiddleware',