Additional CacheService features

[subotic]

• Split all UserADM, ProjectADM, and GroupADM in simple and extended variants. The simple variant will have only IRIs pointing to other admin objects, while the extended variant will return the dereferenced objects. Only simple variant will be cached.
• Add other admin objects to Redis cache (groups, permissions)
• Maybe use Protobuff for serialization
• Expose some cache management commands through the API (e.g., flush DB)