Add improved cache hits for ActiveRecord::Relation collections

[quorak]

Sometimes queries can be very complex and populating models from queries with many includes take
up the majority of time. After checking the cache, it is than obvious populating all these models was not
necessary as we have a hit. Why not checking the cache with a simple query to build the cache-key
and only fire the complex query for those models, that do not exist in the cache?

For these cases you can paste the ActiveRecord::Relation and cache_collection! will:

1. unscope all includes for the initial query to build all cache_key's (make sure you use joins for
   statements that are use in where)

2. gets the result from cache with Rails.cache.read_multi for existing cache_key hits

3. gets all missed hits from the database with complex includes and all fields

4. builds the block with all data

5. uses Rails.cache.write_multi if available to populate the cache with the missed values

   json.cache_collection! Post.includes(:author) do |post|
   json.partial! 'post', :post => post
   end