Make BatchLoader work with the latest GraphQL versions

[exAspArk]

TL;DR

• If you use graphql gem version 1.8.6 or lower, you can continue using BatchLoader.
• If you use graphql gem version 1.8.7 or greater, you need to replace BatchLoader.for with BatchLoader::GraphQL.for to make it work within your GraphQL resolvers (methods).

---

Since version v1.0.0, BatchLoader behaves as a lazy object. It only starts batching when it needs to resolve the values:

batch_loader = BatchLoader.for(1).batch(default_value: :foo) { puts "batching..." }
# => #<BatchLoader:0x140623647733240>

batch_loader.class
# batching...
# => Symbol

puts batch_loader
# foo
# => nil

At the same time, graphql-ruby gem implements a general adapter interface for different lazy execution mechanisms. This interface relies on an instance class method to detect whether an object is lazy or not. Unfortunately, since calling batch_loader.class tries to resolve the values, it is being resolved with N+1 queries by graphql-ruby gem itself.

batch_loader.class
# batching...
# => Symbol

I suggested a few other potentially more flexible solutions for graphql-ruby to detect lazy objects such as duck typing or using explicit arguments. But it looks like it won't be implemented. To fix the issue BatchLoader started wrapping BatchLoader objects with PORO (plain old ruby objects) by using graphql-ruby instrumentation.

This simple BatchLoader object wrapping stopped working since graphql-ruby version 1.8.7. This version introduced a breaking change and started detecting lazy objects before the instrumentation.

In order to fix the compatibility again, this PR introduces BatchLoader::GraphQL - a simple PORO with extra 20 lines of code to wrap BatchLoader without relying on graphql-ruby instrumentation. For example:

class PostType < GraphQL::Schema::Object
  field :user, UserType, null: false

  def user
    BatchLoader::GraphQL.for(object.user_id).batch do |user_ids, loader|
      User.where(id: user_ids).each { |user| loader.call(user.id, user) }
    end
  end
end

---

Related to rmosolgo/graphql-ruby#1778. Fixes #22, #26, #30.

[maletor]

This created a regression where resolvers that do not return a BatchLoader::GraphQL fail to call BatchLoader::GraphQL calls that the resolver uses.