add in-memory cache

Hello @don-neufeld,

Please review the following commits I made in branch 'artem-in-memory-cache'.

352be54a520e714c7b32f558e40e1006424b515e (2013-07-29 23:03:50 -0700)
add in-memory cache

8917ee2bbd0cd4732c6fc4b9ea0c97543cfaae9d (2013-07-29 23:03:28 -0700)
look in further caches for missing items, add mset
if the first available cache doesn't have an item,
it should pass on the request to further caches
to make sure that the db request is necessary.

`mset` was not part of the CacheInstance interface
but it was set up in all of the cache instances.


R=@don-neufeld