Respect physical data placement in parquet iterators

`parquet.List` of structs colocates fields on the same page. This means that we should never create an individual iterator for each of the columns in such cases ([example](https://github.com/grafana/pyroscope/blob/main/pkg/phlaredb/sample_merge.go#L120)): in fact, we fetch same pages repeatedly.

In addition, parquet reader issues a read operation for every ReadBufferPage size (+ page/group bounds), which prevents efficient streaming of data ranges from object storage.