Close parquet.Rows on Flush

I stumble on this recently but one place we don't close the Rows from `parquet.RowGroup`. We should definitively do this to avoid leaking ressources.

This is happening when flushing all rowgroups

https://github.com/grafana/pyroscope/blob/next/pkg/phlaredb/profile_store.go#L373

We could either close in that function or push down closing into the inner tree used for merging.