Use elements from cache in ElementsWorker.run

Depends on !80 (merged)

The goal is to avoid the RetrieveElement request. This should break some stuff (notably open_image but that's good: it will be only used by worker with use_cache=True). Do not aim for backward compatibility here.

The method list_elements should take into account the cache presence, and give CachedElement instances when there are any with initial=True.

To clarify:

1. if use_cache=True
2. AND there are some elements in DB with initial=True in the database
3. then return a list of those elements instead of the JSON list

process_element would then use the CachedElement instance. It is the responsibility of the high level worker to deal with the cached model

Regarding open_image, it might be useful to simply add an open_image(*args, **kwargs) proxy method towards image.open_image (no IIIF manipulation, the correct url should be given by the backend)