Update sets inference manager to inherit from InferenceManagerBuffered (#5007)

Update sets inference manager to inherit from InferenceManagerBuffered (#5007)

This updates the inference manager of sets to inherit from the standard base class InferenceManagerBuffered.

It matches that base class almost exactly, with cosmetic changes.

Notice that sets currently has a design issue: the state object sends lemmas and hence currently requires a pointer to the theory. This will be untangled in a future PR.

This PR makes a few very minor changes to the behavior, including not to use the "preprocess" property for subtype lemmas.