Optimize rights lookup in list requests

[htdvisser]

Summary

We should optimize the rights lookup in list requests to reduce the number of DB roundtrips.

See also https://github.com/TheThingsIndustries/lorawan-stack/issues/1393#issuecomment-683651932

What is already there? What do you see now?

The current implementation checks the rights of the caller for each result in a list request, to make sure we don't return fields the caller doesn't have access to. If uncached, each of these rights checks results in a database roundtrip.

What is missing? What do you want to see?

Instead, we should investigate if we can query the rights for the entire page in a single DB roundtrip.

Environment

v3.9

How do you propose to implement this?

• Define a rights.BulkFetcher interface that is implemented by the IS's DB fetcher
• Add rights.Preload(ctx context.Context, ids ...ttnpb.Identifiers) error

How do you propose to test this?

Use tracing to (no longer) see the many queries.