cubicweb #933769 Entity cache related oddities [open]

Revealed by the preview cube, most easily reproduced by commenting out the call to _clear_related_cache() in it's file, and running test_related_edited test.

Some work on it show that an entity can have in its own "related" cache, entity instances that are not stored in the request eid cache, which cause some problems with the preview cube (that rely on request eid cache being the only source of entity instances to patch download_url of idownloadble entities).

Some more work show that the editcontroller does not use the request eid cache presently, and using it to get/ set entity at the beginning of its edit_entity method, it leads to some more problems, because a modified entity is not edited by itself as a call to _update_entity updates the DB via a rql request.

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