CLC-3274 Replacing DelayedCacheCleaner with smarter conditional cache fetch

https://jira.ets.berkeley.edu/jira/browse/CLC-3274

DelayedCacheCleaner caused server lockup and thread contention, so I nixed it in favor of a more immediate strategy: Introduce a smarter cache fetch method that won't write to the cache if there was an exception. All proxies except Google now use this idiom. Google proxies still don't cache at all and don't use the generic error handling, that's CLC-3234, for future work.