fix: fix threadsNum in AsyncBackup while handle backup

[mlycore]

Type of change:

• [x] Bugfix
• [ ] New feature provided
• [ ] Improve performance
• [ ] Backport patches

What this PR does / why we need it:

fix #455

Pre-submission checklist:

• [ ] Did you explain what problem does this PR solve? Or what new features have been added?
• [ ] Have you added corresponding test cases?
• [ ] Have you modified the corresponding document?

[sonarcloud[bot]]

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