Refactor storage package to focus on how to update instead of what to update

[ariefrahmansyah]

Is your feature request related to a problem? Please describe.

Model Endpoint Storage's Save method in the storage package has a logic on what and how to update both model endpoint and model version endpoint. This could lead to confusion on how to implement service and storage layer in the future.

Related discussion: #483 (comment)

Describe the solution you'd like

• A clear separation and guideline for service and storage layer's responsibilties
• Refactor storage layer to be more responsible on how to update the data and let service layer decide what to save