=str #15948 Make Broadcast cancel upstream when all downstreams have canceled

Fixes #15948
broadcast marks all outputs, and then says AllOfMarkedOutputs to indicate that broadcasting needs all of the outputs to be available (we don't have a default output buffer anymore). Now there is two ways to handle cancellation, one is that broadcast is done if any of the outputs has been cancelled -- since all the outputs are marked, but there is at least one cancelled among them, broadcast would finish. Another alternative is to unmark the cancelled output, so now AllOfMarkedOutputs mean all the remaining outputs (this is the original tee behavior). This flag is supposed to switch between these two behaviors (not exposed to user API so far).