refactor: abstract TransformSortMergeBase to unify merge sort logic.

[RinChanNOWWW]

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Summary

Use TransformSortMergeBase to unify the most logic of TransformSortMerge and TransformSortMergeLimit.

This is also a pre-task of implementing external sort.

TransformSortMergeBase will be wrapped in AccumulatingTransformer. It makes the transform can either outputs block or not while processing the input block:

• Common path: not output blocks until on_finish.
• Spill path: output block with spilling information in its block meta.

Tracking in #13889

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