Refactor SortExec's buffered batches for better code readability

[2010YOUY01]

Is your feature request related to a problem or challenge?

Reference #15355 (comment) #15355 (comment)

The high-level execution logic of SortExec is described in https://github.com/apache/datafusion/blob/main/datafusion/physical-plan/src/sorts/sort.rs, now a single field in_mem_batches is used to represent buffered data in different stage: during different time, it can be interpreted as either unordered input batch, or globally sorted batch:

datafusion/datafusion/physical-plan/src/sorts/sort.rs

Line 229 in 5210a2b

in_mem_batches: Vec<RecordBatch>,

This approach has poor understandability and is also error-prone, see the idea in reference discussion for improvements.

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[2010YOUY01]

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