Improve must_consolidate setting in single-time dataflow refinement

[vmarcos]

As observed in #18546 (comment), there is an opportunity to set the must_consolidate field of monotonic operators to false on single-time plans where the input to the operator is already consolidated upstream. For example, if the input given to a monotonic top-k operator originates from a reduce operator, then we know that consolidation in the monotonic operator is unnecessary. The reduce operator already arranges its output, implicitly consolidating it; the latter results in a monotonic stream in a single-time context. Similarly, inputs originating from indexes or ArrangeBy nodes would become monotonic in a single-time context, and remain so if other intermediate operators do not break monotonicity.

This issue is a request to revisit the single-time dataflow refinement in finalize_dataflow that is introduced by #18546 and introduce additional reasoning to selectively set must_consolidate to false whenever possible, as explained above.