Previously we used a `shouldReconcile` method in order to decide whether
a response should trigger a reconciliation of the frontend ember-data
'source of truth' or not. It's a lot nicer/clearer if this 'flag' can be set
alongside the HTTP request information, moreover we almost have the same
functionality in `If-Range`/`Partial Content` HTTP functionality.
Here we partly follow this HTTP semantics but use a custom `X-Range` header
instead.
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