Directly reloading a hasMany with links should trigger only one request

Fixes #2379 

cc/ @jcbvm @igorT 
I was considering the many array like a model, and a model has the same kind of mechanism.
It's handled both in the [states](https://github.com/emberjs/data/blob/master/packages/ember-data/lib/system/model/states.js#L496), and in the [store](https://github.com/emberjs/data/blob/master/packages/ember-data/lib/system/store.js#L516). I felt like I should do the same for the many array, and I must admit that it was the easiest way.