Currently, the #pluck method returns all results at once as an array. This is problematic when working with large result sets.

I'd like to have something like Criteria#each, where pluck results are progressively yielded as they are fetched from the database (using an Enumerator). I propose to call this method #pluck_each.

I've started a draft PR here: https://github.com/mongodb/mongoid/pull/5332