Read full permission and path snapshot from SentryDB and make the update available for NN plugin upon requests. In detail:
- Added Path/PermissionImage classes to represent corresponding Path/Permission snapshot read from DB.
- Refactor full snapshot retriever APIs in SentryStore to become a single transaction to ensure snapshot consistency.
- Added Path/PermissionImageRetriever classes to retrieve Path/PermissionImage from DB and convert to corresponding Path/PermissionUpdate, which later would be consumed by NN plugin.