=str #16233 adds ReadAll for FlexiMerge

With one discussion point - what should we expose in java/scala apis? `Array` in both - in scala then we cant pattern match on it, but maybe that's not important anyway hm

Refs #16233
I don't think it is enough to check `isCancelled`, it can also be completed (depleted) from upstream
I think the right condition is in `def markInputs`, i.e.

    inputMapping.contains(id) && !inputBunch.isCancelled(id) && !inputBunch.isDepleted(id)