engine-table aggregations need cleanup

[rcaudy]

As a developer, I want to be able to easily trace through the code paths for aggregations to ensure implementation correctness.
Once #1618 merges, we'll have the tools for a major cleanup pass to eliminate the last vestiges of the AggregationStateFactory objects.

In practice, we should be able to go from table-api Aggregation objects to engine-table IterativeChunkedAggregationOperator objects via a single mechanism (rather than two duplicative ones) with the business logic concentrated and easily inspected.

We should also eliminate legacy configuration objects (AggregationElement FKA ComboBy).

Lastly, we should use table-api Aggregation objects as the memoization key for aggregations.

[rcaudy]

I need to sort out format column dropping in aggBy (aggAllBy is good). Remember to annotate overloads on Table for new TableOperations, too.