Add a option to save Llama index automatically

[golbin]

I add an option LLAMA_INDEX_AUTO_SAVE as an environment variable to save Llama index automatically.

If the below option is set, the datastore save index automatically when index is added or deleted

export LLAMA_INDEX_AUTO_SAVE=True

I tested some prototypes but it's very annoying I have to upset many documents every time.

This option is very useful to test prototypes.