A parallel scope completes whoever accounts for its last iteration

[noha]

The loop is over once every iteration is accounted for - arrived at the scope join, filtered by a condition, or taken back (#atEnd). Only the scope join's own step used to close the scope; a withdrawal, a filtered last iteration or an empty input popped the executor and left the flow behind the loop unrun while the instance read #done.

FBParallelExecutor>>completeScope is now the one way the scope closes: from the join's step, from #continue after a withdrawal or a filtered iteration, and from the fork's behavior for an empty input. The scope join's activation carries one value per iteration - nil where none arrived - and goes on behind the join on a token of the fork's strand, in the fork's scope. The loop completes with nothing as well: an empty input, every iteration filtered or every iteration taken back leave the flow behind the loop to go on with an empty result - the loop is one activity of the flow, not a join of independent strands. A body whose every path ends at an end event has no join to go on from; the scope closes and the fork's strand is over, as before.