CLC-3274 Feature flag for delayed cache cleaner, defaulting to enabled

https://jira.ets.berkeley.edu/jira/browse/CLC-3274

Experimental to see if DelayedCacheCleaner is responsible for the increase in 502s seen in the load test. 

The flag defaults to enabling the feature. On qa's production.local.yml, I've added Settings.features.delayed_cache_cleaner: false, which has no effect right now, but will become effective if this PR is merged.