[Discussion] Refactor worker class

[joneswong]

    It is ok to inject those feature engineering procedures into an FL course in this way, but we shall change the instantiation of workers to a better (more general and unified) way. Actually, it is quite confusing to wrap a worker by feature engineering-related wrapper. Feature engineering is just a tiny step in an FL course, which doesn't change a worker significantly. One usual way to instantiate a worker from a collection of such pluggable behaviors is to use factory pattern I guess.

Originally posted by @joneswong in #426 (comment)