Add function to datastore to return the unused indexes at the end of the tests

[riannucci]

Can be useful to assert-is-empty to ensure that you're not indexing things that you don't need. This should include both compound and simple (builtin) indexes (whose cost can be avoided by listing them as noindex).

[vadimsht]

It may be more useful to return used compound indexes instead (+ auto register index mode). So that unit tests may suggest what index to add to index.yaml.

[riannucci]

Ah, right you mentioned that before. Forgot that part.