Corrections after reworking backup/restore synchronization #3

[vitlibar]

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Follow-up to #70027 and #71715 and #71912

[vitlibar]

This change is to fix the failure of test_backup_restore_on_cluster/test_cancel_backup.py::test_long_disconnection_stops_backup.

[robot-ch-test-poll1]

This is an automated comment for commit e00128c with description of existing statuses. It's updated for the latest CI running

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[vitlibar]

This change is to fix the failure of test test_backup_restore_on_cluster/test_cancel_backup.py::test_cancel_restore.

[vitlibar]

I also added more comments.