Local config cache feature

Caches accesses to the main cache, only for Configs. This is a tradeoff between responsiveness (of the changes in Admin UI) and performance (avoiding network round trips)

- Usage : set `memcached.configLocalBuffer` with a value in milliseconds.
- Recommended: a few seconds in dev/QA, more in production.
Got it. GuavaCache's `CacheBuilder.newBuilder()` forces us to use `Object` as values it seems. Never mind then, not worth the trouble / ugliness of hacking around this :)