Support multiple unique fields in dgraph GraphQL

@rajas I just wanted to give an example of a perfect use case for this. While you are building graphql facets, there is a need for this in basic graphs.

Let’s say a user reviews a movie, there should be only one review:

Obviously, it would be better to have a unique restraint like in SQL:

ALTER TABLE `Rating` ADD UNIQUE `unique_index`(`user`, `movie`);