opt: make ReleaseTimeout() more efficient in waiting workers to exit

[panjf2000]

Also, this PR utilizes embedding structs to eliminate duplicate fields of Pool and PoolWithFunc.

[codecov]

Codecov Report

Attention: Patch coverage is 87.09677% with 12 lines in your changes missing coverage. Please review.

Project coverage is 90.52%. Comparing base (3ffd3da) to head (b9e7aaf).

Files	Patch %	Lines
pool.go	85.36%	5 Missing and 1 partial ⚠️
pool_func.go	86.36%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #329      +/-   ##
==========================================
- Coverage   91.32%   90.52%   -0.81%     
==========================================
  Files          11       11              
  Lines         761      802      +41     
==========================================
+ Hits          695      726      +31     
- Misses         45       53       +8     
- Partials       21       23       +2     

Flag	Coverage Δ
unittests	90.52% <87.09%> (-0.81%)	⬇️

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