forwarder: close goroutine leak when block in accept

[HuSharp]

related tikv/pd#7782

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[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 24.17%. Comparing base (5e932ec) to head (cce4e52).
Report is 16 commits behind head on master.

❗ Current head cce4e52 differs from pull request most recent head f3b363d

Please upload reports for the commit f3b363d to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1644      +/-   ##
==========================================
+ Coverage   23.68%   24.17%   +0.49%     
==========================================
  Files         177      173       -4     
  Lines       16055    15725     -330     
==========================================
- Hits         3802     3801       -1     
+ Misses      11968    11638     -330     
- Partials      285      286       +1     

Flag	Coverage Δ
backend_integration	9.29% <91.66%> (+0.30%)	⬆️
backend_ut	26.28% <54.16%> (+0.09%)	⬆️

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[HuSharp]

@shhdgit @baurine PTAL, thx!

[baurine]

@mornyx please help review , thanks~

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