feat: add support for ORM `fetch_mode_subselect_batch_size` configuration #1742

Job	Run time
PHPUnit (7.4, highest, stable, false)	34s
PHPUnit (8.0, highest, stable, false)	24s
PHPUnit (8.1, highest, stable, false)	25s
PHPUnit (8.2, highest, stable, false)	35s
PHPUnit (lowest, stable, 7.4)	33s
PHPUnit (5.4.*, highest, 8.0)	25s
PHPUnit (6.*, highest, 8.3)	36s
PHPUnit (8.3, highest, stable, true)	20s
PHPUnit (8.3, highest, dev, true)	33s
PHPUnit (8.3, highest, dev)	33s
Upload coverage to Codecov	0s
	4m 58s

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