text: Title: Use of DataLoader to solve exorbitant query times
  Body: There's a dramatic reduction in query performance when using GraphQL with graphene-mongo, as opposed to a regular REST API doing something like `list(collection.objects.all())` and sending the entire document set. From 300ms to over 6 seconds to retrieve one field from 100 elements.
  
  I believe that using the DataLoader pattern is the expected solution to the N+1 problem, but actually attempting to use it will cause a runtime error
  ```
  graphql.error.located_error.GraphQLLocatedError: Data loader batch_load_fn function raised an Exception: AttributeError("type object 'Person' has no attribute 'objects'",)
  ```