backingStoreFindMulti's shouldn't worry about writing to cache

FeatureIndex::findMulti is the one doing the talking to cache and requesting
fallback to storage if not found in cache; he's the one that should be doing the
saving to cache.

Also make failure return values all be array() instead of the false/array()
split. FeatureIndex::findMulti doesn't do anything with false anyway.

Change-Id: I804d58201d784cf5325d4d90b3bc3a0999d3d3f8
Fallback data should not just be added to cache, but CAS'ed

Chances are quite low of another request writing new data while we just read the
stale data from DB, but they exist :p

Change-Id: Idf0c8f32171baca9412901e05598b3da4417f233
Merge "backingStoreFindMulti's shouldn't worry about writing to cache"
Merge "Fallback data should not just be added to cache, but CAS'ed"