Follow the "convention over configuration" paradigm the index startup mode should be default to "auto".

This will ensure that the index is vaild and aligned to the cache. The user will get a valid configuration by default and WARN messages (or reject the config) if something is inconsistent!
As well the index should default to "in memory" (local heap) if the cache is not persistent.

As result

• default cache without persistence and index, will use a local heap index without action at start
    - If the index is explicit set to persistent start action should be clear
• persistent cache with index, will use persistent index without action at start
    - If the index is explicit set to non-persistent start action should be rebuild (maybe with WARN or INFO message because of performance)
• ANY manual configuration should show WARN messages if not have a valid combination
    - i.e. persistent cache non persistent index with startup-mode=none||purge
    - non persistent cache with persistent index
    - non persistent cache with startup action rebuild -> unnecessary