Skip JSON and cache generation

https://redmine.teklia.com/issues/11528#note-14

Requires backend#1961

When the skip_elements_json option is True on the process, the InitElementsWorker.list_process_elements and InitElementsWorker.dump_chunks methods should not be called. This will cause the worker to stop generating JSON files and SQLite cache databases, and it will only wait for worker activities to be generated.