object cache deletion refactoring, part 2

these changes concern how objects which interact with Cachex are managed. to ensure we do not have zombie objects returning, we should never ever directly delete the objects with Repo.delete, and instead use object-specific deleters.

in this MR:

• Object.delete() cleanups pointed out by lain
• User.invalidate_cache() now deletes zombie user_info data.
• User.delete() now returns {:ok, deleted_user} to match Object.delete().
• Object cache is now split from User cache.
• Regression tests for user deletion + cache interactions.