opt: renovate the concurrency management of gnet engine

[panjf2000]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 8 lines in your changes missing coverage. Please review.

Project coverage is 79.40%. Comparing base (451f015) to head (e96bcf4).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
acceptor_windows.go	0.00%	0 Missing and 2 partials ⚠️
engine_unix.go	93.54%	0 Missing and 2 partials ⚠️
engine_windows.go	91.66%	0 Missing and 2 partials ⚠️
gnet.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #663      +/-   ##
==========================================
- Coverage   79.58%   79.40%   -0.18%     
==========================================
  Files          21       21              
  Lines        2537     2515      -22     
==========================================
- Hits         2019     1997      -22     
  Misses        398      398              
  Partials      120      120              

Flag	Coverage Δ
unittests	79.40% <90.90%> (-0.18%)	⬇️

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