need to separate subscribe logic for Dubbo ops and dubbo app clearly.

right now, subscribe logic in all registry impls is complex because we mix the duties of dubbo ops and dubbo app together.

Start by locating the subscribe logic in all registry implementations and trace where Dubbo ops and Dubbo app responsibilities are currently mixed. Compare the duties handled by each path before deciding how they should be separated. Done means the subscription responsibilities are clearly distinct across the registry implementations.

Written by the indexing model from the issue text.