Fix tunneling of CF decisions to inactive graph regions in multi-parent configurations

This is a scalable bottom-up alternative to !857 (closed).

• added a scout visitor to walk through a graph lineage of a node to deduce if the node is considered active through that lineage (the scout is used for multi-parent nodes before the main - promoting - traversal is triggered);
• added test with an algorithm shared by two CF branches one of which early terminates;
• added view-awareness to subgraph scouts and removed CFplusViewBug test (provided by @bwynne).

This MR is based on !861 (merged) (one commit on top), please jump to that one first.

@bwynne, FYC.