AI: Celery workers lose their cache after an exception causing many hits to S3

Fallback to file-based cache for classifier data
Add info-level logging for cache hits/misses

[TIM-652](https://edx-wiki.atlassian.net/browse/TIM-652): Celery workers lose their cache after an exception causing many hits to S3

@stephensanchez This is now ready for review :)
Yeah, I think it will be fine.  The cache also has a timeout, so as long as the cache keeps getting accessed, unused classifiers will be deleted.