Automatically detect use_cache initial value

The base worker should now be able to detect if use_cache is True (we stick to False by default).

BaseWorker & ElementsWorker should not have a use_cache in their constructor, but a support_cache=False instead.

Then self.use_cache becomes True, if (and only if):

• support_cache is True
• an sqlite database is available in the parent tasks

The ML workers would then just have to declare if they support cached elements alongside normal elements