Use `records` for joined resources

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• Problem/bug report.
• Feature request.
• Request for support. Note: Please try to avoid submitting issues for support requests. Use Gitter instead.

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• I've searched for an existing issue.
• I've asked my question on Gitter and have not received a satisfactory answer.
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Description

Current:

In version 0.9 and earlier JR relied on the Rails eager loading using the ActiveRecord includes. In a typical Rails app this feature works great, but we ran into a lot of edge cases and performance compromises when auto generating the queries JR needs for complex requests - especially ones with complex includes. In addition the complexity grew with caching.

With this approach we recommended that related resources be controlled using relation_name on the relationship. This relation was used in the includes and provides a way to control the related resources.

With version 0.10 the approach was changed and we no longer use rails eager loading. The relation_name on the relationship can still be used for relationship filtering.

There is also the option of using the apply_join callable on the relationship.

Both of the above options however require explicitly wiring up the appropriate code on each relationship.

Proposed change:

When joining related records in ActiveRelationResource.apply_join we have to opportunity to use ActiveRecord's merge method to apply the related resource's records logic. The implication of this is all accesses to a Resource will use the same records applied logic whether coming to the resource as the primary resource in a request, as an included resource, or through the related link of the parent resource.

I believe this one change should drastically simplify resource authentication and opens up some further opportunities to further simplify most resources.