Provide a way to get notification when results of a query change

If one wants to observe changes to a set of documents matching a query, you can use fullDocument to match on document's content and get notifications for changes made to those documents. But this is limited in use by the fact that there is no notification when a document stops being matched by the query. See https://github.com/meteor/meteor-feature-requests/issues/158#issuecomment-339376181 of the issue as we are investigating how to use Change Streams to support reactivity in Meteor.