Database caching isn't implemented.

Example crash when running Wagtail test suite :

AttributeError: 'NoneType' object has no attribute 'execute'

django-mongodb doesn't use cursors and isn't able to execute SQL, so a custom cache backend is needed.

Workaround is to disable use of database caching in the tests: https://github.com/aclark4life/django-mongodb-cli/blob/36515d54ab94ca82085a1c898d1b478c1c423e10/test_settings/settings_wagtail.py#L182-L190