Split query iterators over different row groups

[simonswine]

We currently ran a whole block through the query iterator:

https://github.com/grafana/phlare/blob/4d7eb651305bad4c44adbbecec30ee8c8d49b6d9/pkg/phlaredb/block_querier.go#L983

This should be called per rowGroup and ideally respect context cancellation