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Make `partialize_agg()` support parallel query execution. To make this work, the finalize node need combine the individual partials from each parallel worker, but the final step that turns the resulting partial into the finished aggregate should not happen. Thus, in the case of distributed hypertables, each data node can run a parallel query to compute a partial, and the access node can later combine and finalize these partials into the final aggregate. Esssentially, there will be one combine step (minus final) on each data node, and then another one plus final on the access node. To implement this, the finalize aggregate plan is simply modified to elide the final step, and to reserialize the partial. It is only possible to do this at the plan stage; if done at the path stage, the PostgreSQL planner will hit assertions that assume that the node has certain values (e.g., it doesn't expect combine Paths to skip the final step).