ArakoonResultBackend should allow DataObjects to be stored

[JeffreyDevloo]

Feature description

Using Arakoon as resultbackend made us discard pickling. Putting DataObjects on the current backend will default to using the kombu JSON serializer which will fail. To counter this, we should store some extra descriptor information with it so we re-create the stored object.

However this method will also require some looking at our current API decorators. The TaskViewSet serializes the result as a simple object (list/dict/...). This means that when a task puts a DataObject on the result backend, fetching that won't make it properly serialized through the API. To counter this, one could opt to rewrite the decorators so they can handle different types