opt: streamline pool implementation to reduce duplicated code

[panjf2000]

Also, bump up the minimal required Go version from 1.16 to 1.18.

[codecov]

Codecov Report

Attention: Patch coverage is 91.60305% with 22 lines in your changes missing coverage. Please review.

Project coverage is 89.71%. Comparing base (4f33c6e) to head (7852f9e).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
ants.go	91.66%	14 Missing and 5 partials ⚠️
pool_func.go	86.66%	1 Missing and 1 partial ⚠️
worker.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #350      +/-   ##
==========================================
- Coverage   89.95%   89.71%   -0.24%     
==========================================
  Files          11       11              
  Lines        1005      797     -208     
==========================================
- Hits          904      715     -189     
+ Misses         76       62      -14     
+ Partials       25       20       -5     

Flag	Coverage Δ
unittests	89.71% <91.60%> (-0.24%)	⬇️

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