Bluetooth: Mesh: Refactor Mesh Model Extensions

The existing extension tree does not support all the features that are
defined by the specification (e.g. multiple parents).

This patch approaches this problem by defining a circular single-linked
list of extension models. So for a given model, all models that are on
the same list as that model are in some extension relationship with that
model. All models on a list represent a single connected component of an
extension graph but without defining specific relationships between each
pair of models. This list is used to manage a shared subscription list
as per the Mesh Profile Specification:

```4.2.4 Subscription List

Within an element, each model has a separate instance of a Subscription
List, unless the model extends another model on that element. Instances
of models that extend other models (i.e., all models within an extension
relation tree) shall share a single instance of a Subscription List per
element.
```

Signed-off-by: Michał Narajowski <michal.narajowski@codecoup.pl>