[dataquery] Add ability to retrieve results of previous run from API

[driusan]

This implements the endpoint that is listed as "aspirational" in the dataquery api schema to load the results of a previously run query.

It was originally left out for fear of performance issues but with this implementation performance seems to be comparable to without it.

This does not implement a frontend to reload query results, but the API functionality is useful for i.e. api callbacks that want to process the results of a query without the need to re-run it (and the possibility to get different results.)