Seperate the memory based cache and file based cache in OC_Cache

Both cached have a different use cache, the file based cache is used for things like contact photos and minified css/js files, the memory based cache shouldn't be used for that.

iirc, the is no (longer) anything which uses the memory based cache, I'm planning to create a session handler using the memory based cache.

Also added a memcached backend.

@karlitschek @bartv2