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ARROW-6969: [C++][Dataset] ParquetScanTask defer memory usage

The constructor (via Make) would eagerly create the RecordBatchReader which would consume memory even if the ScanTask was not consumed yet.

This creates a memory usage problem with Scanner users who collect all ScanTask before dispatching them, e.g. by using TaskGroup.

This patch defer the creation of the RecordBatchReader in the `Scan` method of the ParquetScanTask.

Closes #5725 from fsaintjacques/ARROW-6969-parquetscantask-eager and squashes the following commits:

a5a62da1d <François Saint-Jacques> ARROW-6969:  ParquetScanTask defer memory usage

Authored-by: François Saint-Jacques <fsaintjacques@gmail.com>
Signed-off-by: François Saint-Jacques <fsaintjacques@gmail.com>