Fix Connection Cache for ActiveRecord

[ssnickolay]

Problem

Connection cache does not work with ActiveRecord.

Cause (for defects/bugs)

For ActiveRecord this returns GraphQL::Relay::RelationConnection where nodes is instance of ActiveRecord_Associations_CollectionProxy. This is because ActiveRecord is lazy by design and when we call field.resolve_proc.call(obj, args, ctx) there is not a single SQL query in the database. SQL query is executed when we call to_a on RelationConnection level.
So the problem is that currently we put into the cache not loaded ActiveRecord_Associations_CollectionProxy and whenever we have cache hit we still have an SQL query:

DEBUG -- : Cache hit: (gql:::Playground:roles:Playground:playgrounds/726-20191216110227000000)
Role Load (3.6ms)  SELECT `roles`.* FROM `roles` WHERE `roles`.`playground_id` = 726 LIMIT 50

DEBUG -- : Cache hit: (gql:::Playground:roles:Playground:playgrounds/726-20191216110227000000)
Role Load (3.6ms)  SELECT `roles`.* FROM `roles` WHERE `roles`.`playground_id` = 726 LIMIT 50

...

Solution

1. Add ActiveRecord test suite and reproduce the bug.
2. Handle RelationConnection separately and cache paged_nodes when they are loaded.

Notes

This is still not an ideal solution because we cache only paged_nodes, but RelationConnection has other methods (e.g. sliced_nodes_count). But we don't know which exactly will be executed and I don't see how we can handle it without extra cache write.