Remove CacheLookupsMixin

We've seen some issues with cached lookups failing to return records
when running in production.

Refs #65
Remove 'dalli' dependency

It was only needed because CacheWatcher was comparing classes to 
determine the current cache store. Switching to string comparison means
that the host app can require its own cache store.
Merge pull request #66 from osbridge/without_cache_lookups_mixin

Remove CacheLookupsMixin