querying doesn't stop when new query is selected

[AronBuzogany]

In V1, we were able to stop the query as it was processed as a stream, we unsubscribed from the iterator and killed it.
Currently we have to use toArray on the iterator and return a promise to react-admin.

[mvanbrab]

All async operations in SparqlDataProvider.getList still keep on running when selecting a new query indeed, but at least with the current state of the code, this seems harmless for the result of that new query and of the original query, when going back.