GraphQL: Unlock goroutine throttle to match batch capacity (#20146)

We were throttling the amount of concurrent resolvers
at a given time. The idea behind this was to avoid overloading
the database with too many requests.

However, with the introduction of dataloaders, this limitation
actually becomes a bottleneck because all DB calls are actually
batched, so we are unnecessarily throttling the amount of
items that can be processed in a single batch.

The only caveat with this is that now all resolvers
need to backed by dataloaders, or otherwise not be queried
as part of a loop.

In a subsequent PR, we will be removing channel stats
from under channel to be a top-level object to be returned
for a given channel.

```release-note
NONE
```

api4/resolver.go

@@ -313,3 +313,12 @@ func getTeamsLoader(ctx context.Context) (*dataloader.Loader, error) {
 	}
 	return l, nil
 }
+
+// getUsersLoader returns the users loader out of the context.
+func getUsersLoader(ctx context.Context) (*dataloader.Loader, error) {
+	l, ok := ctx.Value(usersLoaderCtx).(*dataloader.Loader)
+	if !ok {
+		return nil, errors.New("no dataloader.Loader found in context")
+	}
+	return l, nil
+}