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.
Might be nice to have a comment describing how to disable the local buffering cache.

From what I can tell, the best way to do that at the moment is to remove this line (the other cache-related configs seem to be toggled by setting a boolean, so I'm not sure if you want to make this match those).