Refactor relationships with links

Placeholder
Fixes http://discuss.emberjs.com/t/requests-triggering-multiple-times/6316
only because you removed `this.hasFetchedLink` but it's being referenced in the `relationship.fetchLinks`(https://github.com/emberjs/data/blob/master/packages/ember-data/lib/system/relationships/relationship.js#L180 ) which is being called by `relationship.reload` on the other hand.