perf: reduce StackedConfigurationModel cache queries

TieredCache is a hybrid request cache + regular cache, where we check
against a Django cache for the first get, but then store that data in a
process-local request cache. ConfigurationModels were already converted
to use TieredCache for performance reasons, but we never converted the
StackedConfigurationModel subclass.

This can reduce the lookups for ContentTypeGatingConfig by hundreds of
network cache calls in a given request (depending on how many exams are
in a course).

This also affects the following subclasses of StackedConfigurationModel,
though I have not measured the precise effects:

* CourseDurationLimitConfig
* DisableProgressPageStackedConfig
* DiscountRestrictionConfig
* DiscountPercentageConfig
* ProviderFilter (for dicusssions providers)
* SelfPacedRelativeDatesConfig

openedx/core/djangoapps/config_model_utils/models.py

@@ -11,14 +11,15 @@
 from enum import Enum
 
 import crum
-from config_models.models import ConfigurationModel, cache
+from config_models.models import ConfigurationModel
 from django.conf import settings
 from django.contrib.sites.models import Site
 from django.contrib.sites.requests import RequestSite
 from django.core.exceptions import ValidationError
 from django.db import models
 from django.db.models import Q
 from django.utils.translation import gettext_lazy as _
+from edx_django_utils.cache.utils import TieredCache
 
 from openedx.core.djangoapps.content.course_overviews.models import CourseOverview
 from openedx.core.djangoapps.site_configuration.models import SiteConfiguration
@@ -157,10 +158,9 @@ def current(cls, site=None, org=None, org_course=None, course_key=None):  # pyli
             no arguments are supplied).
         """
         cache_key_name = cls.cache_key_name(site, org, org_course, course_key)
-        cached = cache.get(cache_key_name)
-
-        if cached is not None:
-            return cached
+        cached_response = TieredCache.get_cached_response(cache_key_name)
+        if cached_response.is_found and cached_response.value is not None:
+            return cached_response.value
 
         # Raise an error if more than one of site/org/course are specified simultaneously.
         if len([arg for arg in [site, org, org_course, course_key] if arg is not None]) > 1:
@@ -235,7 +235,8 @@ def sort_key(override):
 
         current = cls(**values)
         current.provenances = {field.name: provenances[field.name] for field in stackable_fields}  # pylint: disable=attribute-defined-outside-init
-        cache.set(cache_key_name, current, cls.cache_timeout)
+
+        TieredCache.set_all_tiers(cache_key_name, current, cls.cache_timeout)
         return current
 
     @classmethod