[BUGFIX relationships] fix infinite retry bug for failed relationship fetches

[runspired]

This PR unskips tests skipped for landing RecordData and fixes regressions introduced around relationship fetching in 3.5

cc @code0100fun please confirm this resolves #5814 et al.

[code0100fun]

🎉 Thanks for fixing this! It will be awesome to get back on the latest release 😄

This did fix my errors with infinite 404's. However, I noticed a few more errors in my test suite when running with this branch (they do not happen on current master branch). They all center around ember-data relationships and mostly when unloading a new record.

One bug seems to happen when pushing directly into a relationship (ex: aModel.hasMany('child-model').push(data) ) and immediately after calling newChildModel.unloadRecord(). This causes computeds with cache keys like 'aModel.childModels.[]' to re-evaluate but there is a null item in the relationship collection. This one is tricky and I'm not sure how to reproduce in a test.

Another bug I am still working through is that calling newChildModel.unloadRecord() causes the internal model to be in an isEmpty state. Afterwards, calling aModel.childModels calls InternalModel#getHasMany which eventually gets down to PrivateStore#_findEmptyInternalModel and calls _scheduleFetch for a model with a null id. This one is relativly straight forward so I think I can put together a failing test for it.

I still have one other test error to investigate but I'm hoping it is just another manifestation of one of the above issues.

[runspired]

Test failures are the usual suspects (flakey Travis test, odd linting issue when running M3 with newer Data)

[mansona]

Is this going to be backported? I'm just wondering because of the large impact of the bug?

[runspired]

@mansona not likely given it wasn’t reported much despite being an issue for 6 releases and its a harder fix to port. It’ll be in 3.12.

[jakebixbyavalara]

@runspired we're actually experiencing this issue with v3.1.2 (which we upgraded to for the fix for #4963). We're aiming to upgrade rapidly soon, but have a few blocking factors that we have to tackle first.
Do you have any suggestions on patching to at least short circuit the retry loop?

[Leooo]

@runspired any Idea if we can consume Ember data 3.12 with Ember 3.4 ? Trying ED 3.5 upgrade atm and the bug above would be a blocker for us (upgrading to higher Ember versions across our main app + its engines would take us months). There was another blocker bug in ED 3.4

@mansona @jakebixbyavalara interested if you have any ways around this thanks
Thanks

[Mifrill]

@Leooo as for "have any ways around", you can try this way: https://github.com/Mifrill/ember-data-stop-infinite-retry-for-failed-relationship-fetches