feat: add MultiPool and MultiPoolWithFunc

[panjf2000]

No description provided.

[codecov]

Codecov Report

Attention: 40 lines in your changes are missing coverage. Please review.

Comparison is base (27685ba) 94.43% compared to head (5f4149d) 91.28%.

Files	Patch %	Lines
multipool.go	79.16%	15 Missing and 5 partials ⚠️
multipool_func.go	79.16%	15 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #305      +/-   ##
==========================================
- Coverage   94.43%   91.28%   -3.16%     
==========================================
  Files           9       11       +2     
  Lines         737      929     +192     
==========================================
+ Hits          696      848     +152     
- Misses         30       60      +30     
- Partials       11       21      +10     

Flag	Coverage Δ
unittests	91.28% <79.16%> (-3.16%)	⬇️

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