Specialization for handoffs preceeding non-monotone/blocking operators (perf optimization)

[MingweiSamuel]

For example, if we have max() or min(), a preceding handoff would only ever need to store a single max or min element, streaming. But currently we only have VecHandoffs, which store everything. So we can be more efficient. This will probably tie in to using lattice types in handoffs.

[jhellerstein]

Another example: Sort is preceded by Handoff, but we could have a HeapHandoff or something that would do most of Sort, and save rebuffering.

[MingweiSamuel]

@zzlk is found room for optimizing the current vec handoff #480

[jhellerstein]

Required:

• StatelessHandoff doesn't own state, it participates in the state of its surrounding components

Future Optimization Rule:

• Rewrite rules to identify when we can replace Handoff with StatelessHandoff