Add get max size to chain reader.

[nolag]

I'll need this to fully replace codec calls in median. Haven't prototyped the how yet, but seems pretty simple given it can assume there's only one level of slice and everything in it will then be fixed size :).

[reductionista]

Looks like you need to add something like this to config.go:

GetMaxSize(ctx context.Context, n int, itemType string) (int, error) {
	return 0, errors.New("not used for these tests")
}