The $changeStream stage should not be allowed inside a $lookup pipeline, since there is no way to get "all" results from a tailable cursor. Also, while we do not currently allow the user to specify a pipeline for $graphLookup, and $changeStream is banned in views, we should look ahead to ensuring $changeStream is not allowed in a $graphLookup pipeline if we allow users to specify $graphLookup pipelines in the future.