Pass processing step to worker (#779)
* refactor: 💡 move WorkerLoopConfig to the worker's code
* refactor: 💡 rename WorkerFactory, and pass graph
The workers factory does not only create dataset-based workers, so it
should not be named DatasetsBasedWorkerFactory. Also, to reduce the
code, we pass the processing step to the worker at its creation, instead
of getting it afterwards.
* refactor: 💡 simplify signature of WorkerFactory
