This allows the sender of the command to rely on NamespaceNotFound if the collection has been dropped, even if a collection with the same name was later re-created.

• Change AggregationRequest::_nss from a NamespaceString to a NamespaceStringOrUUID.
• Change the return type of AggregationRequest::parseNs() to NamespaceStringOrUUID and add a case for UUID similar to CommandHelpers::parseNsOrUUID().
• Change runAggregate() to get the NamespaceStringOrUUID. One approach would be to rename and change the return type of AggregateRequest::getNamespaceString().

It should be fine for the AutoGetCollectionForRead in DocumentSourceCursor::loadBatch() to continue to using a NamespaceString rather than a UUID because the PlanExecutor is guaranteed to be killed (and for the aggregation pipeline to eventually error) if the collection is dropped while the cursor is being iterated.