Fix cache key generation to use class name and module
Fix cache key generation to use class name and module

This was stringifying the class object directly, resulting in cache keys
like `:1:<class 'xblock_django.models.XBlockConfiguration'>.xblock_django.api.deprecated_xblocks`
which then cause breakage when Django 2.2.13 validates the keys.

(cherry picked from commit 0a3504334b0ab30b51bd8ea90306f943827d9586)

edx-solutions/edx-platform,appsembler/edx-platform,mitocw/edx-platform,mitocw/edx-platform,edx-solutions/edx-platform,appsembler/edx-platform,mitocw/edx-platform,edx-solutions/edx-platform,appsembler/edx-platform,appsembler/edx-platform,edx-solutions/edx-platform,mitocw/edx-platform