ENH: stop using daemon=True for subpool

[qinxuye]

What do these changes do?

Related issue number

Fixes #xxxx

Check code requirements

• tests added / passed (if needed)
• Ensure all linting tests pass

[codecov]

Codecov Report

Attention: Patch coverage is 97.29730% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 88.52%. Comparing base (2a041ae) to head (5ba4fe0).

❗ Current head 5ba4fe0 differs from pull request most recent head 1a58569. Consider uploading reports for the commit 1a58569 to get more accurate results

Files	Patch %	Lines
python/xoscar/backends/pool.py	95.83%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
- Coverage   89.12%   88.52%   -0.61%     
==========================================
  Files          48       47       -1     
  Lines        4010     3948      -62     
  Branches      764      757       -7     
==========================================
- Hits         3574     3495      -79     
- Misses        351      365      +14     
- Partials       85       88       +3     

Flag	Coverage Δ
unittests	88.44% <97.29%> (-0.51%)	⬇️

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