This separates out the responsibility of pagination from the relation,
and is necessary to test resolvers using `Resolver.single` if they
return an offset paginated relation.

The reason for this is that the `single` resolver calls `super.first`
(more or less), but on a connection, `#first` is the value of the
`first:` argument, not the first item in the collection.

For `BaseResolver.single` to work correctly with offset pagination, we
need to return `relations` from `resolve`, not connections.