CLC-4102 Reduce (& do not encourage) redundant caching

https://jira.ets.berkeley.edu/jira/browse/CLC-4102

* Gets rid of some excess caching and excess cache-updating. Now that the service has scaled up so much, we need to reduce memory, CPU, and data-source consumption as much as we can.
* Makes it easier (I hope) to add cache-consuming features one step at a time as needed (CachedFeed + LiveUpdatesEnabled + JsonCacher + FreshenOnWarm), rather than giving away the store by default.
* Also makes it much easier to combine Live Updates checking with normal cache lifespans (including short lifespans when a data source suffers a temporary glitch) via the LiveUpdatingProxy mix-in.

This is massive, obviously, and I won't be surprised if the remaining team considers it too risky for the current short-handed sprint. But we can at least start collecting comments, testext failures, etc.
Because LiveUpdatingProxy is an additive mix-in (rather than a subclass), it is never included independently but instead always needs to come after an "include Cache::LiveUpdatesEnabled". As a result, all the LiveUpatingProxy classes are also in the LiveUpdatesEnabled classes list and get picked up by UserCacheExpiry.

BTW, I'm not crazy about the module name, which doesn't seem as intuitive as the other mix-ins. Any suggestions? (Or maybe I'll gain inspiration by Monday.)