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.
This should be the minimum of `exp` and `localCacheDuration`