N-way merge first attempt

**DO NOT MERGE**

*This is an early preview with commented out lines, printlns, FIXMEs, bugs and whatnot.*

N-way merge and infrastructure for more complex fan-in operations. Currently following a strictly layered materialization that does not allow cycles. 
Not really, but yes similar. I am thinking of standardising an id field so that we can unify all these messages. And I will reinvent SubstreamOutputs, too :)

(yes horrible, I know, I need to figure out something for this long-term apart from the actorless model that has no such problem ;) )