[PERF-344] Add versioning of cached course assets to allow graceful cache invalidation

When releasing the versioned assets work, we stumbled on a problem with old pickled
versions of the StaticContent objects residing in cache, which triggered a bug in the
code. Not wanting to blow away all cached items, we ended up having to revert and add
in some backwards-compatible helper code to ease the transition.

With this, we'll now utilize the version argument that Django's caching interface
allows, in conjunction with a constant value that can be modified when breaking changes
are being made, to let us gracefully ignore older cached course assets.

cms/djangoapps/contentstore/tests/test_core_caching.py

@@ -1,4 +1,7 @@
-from cache_toolbox.core import get_cached_content, set_cached_content, del_cached_content
+"""
+Tests core caching facilities.
+"""
+from contentserver.caching import get_cached_content, set_cached_content, del_cached_content
 from opaque_keys.edx.locations import Location
 from django.test import TestCase
 

cms/djangoapps/contentstore/views/assets.py

@@ -10,7 +10,7 @@ from django.views.decorators.http import require_http_methods, require_POST
 from django.conf import settings
 
 from edxmako.shortcuts import render_to_response
-from cache_toolbox.core import del_cached_content
+from contentserver.caching import del_cached_content
 
 from contentstore.utils import reverse_course_url
 from xmodule.contentstore.django import contentstore

cms/djangoapps/contentstore/views/tests/test_transcripts.py

@@ -13,7 +13,7 @@ from django.test.utils import override_settings
 from django.conf import settings
 
 from contentstore.tests.utils import CourseTestCase, mock_requests_get
-from cache_toolbox.core import del_cached_content
+from contentserver.caching import del_cached_content
 from xmodule.modulestore.django import modulestore
 from xmodule.contentstore.django import contentstore
 from xmodule.contentstore.content import StaticContent