Stateful apply rule for MyriaX

[billhowe]

Need to stream all results from a partitioned dataset through a single worker for stateful apply to have the correct semantics.

OR notice that the entire dataset is already on one worker

AND EVENTUALLY inspect the update function to determine that it can be computed algebraically in parallel as a hierarchy. (This one is hardest, but the best)

[senderista]

This still doesn't address the ordering issue, right? Do we need to require an ordering on the input to StatefulApply (which would require global ORDER BY support), or do we just assume that the update function is commutative?

Also see #158