Better handling of unique and sparse index constraints

- `:sparse` is now always set (no harm in setting it, in theory speeds up some edge cases where `_slugs` is unset on a large number of records)
- `:unique` is now always set EXCEPT for an edge case related to paranoid docs. Previously we were not setting `unique` when `by_model_type` was true, however, I cannot think of any reason not to set it in this case.