[WIP] Fix extra db hit with belongs_to association

[beauby]

As mentioned in #1100, belongs_to associations currently load the record instead of using the association_name_id attribute when available (i.e. with ActiveRecord).
The explicit calls have been fixed, but the way things are, the records are actually loaded when building the associations anyways. A satisfying fix would be to lazify that part.

This PR is built on top of #1103 (which was initially intended to provide an easy way to fix #1100).

[NullVoxPopuli]

rebase needed.

@beauby, resource_identifier_type_for is a useful method - I should see how the json adapter works / doesn't under the same scenario of belongs_to :author, class_name: User.name

[beauby]

I'll rebase this eventually but currently it requires more work during the phase when the serializer actually gets built in order to fix the issue.

[sheldonbaker]

Has there been any recent activity on this or #1100? I'm currently getting bit pretty hard by this issue. :)

[NullVoxPopuli]

@beauby can you rebase this? we recently had another issue open about this. #1552

[beauby]

This doesn't fix it as the problem lies upstream + needs discussion because the real fix implies tighter coupling with AR. Closing this for now.