add Process\Pool::detach()

[matyhtf]

No description provided.

[codecov]

Codecov Report

Merging #4221 (357c305) into master (8ce5041) will increase coverage by 0.01%.
The diff coverage is 25.80%.

@@            Coverage Diff             @@
##           master    #4221      +/-   ##
==========================================
+ Coverage   50.18%   50.19%   +0.01%     
==========================================
  Files          71       71              
  Lines       14268    14294      +26     
==========================================
+ Hits         7160     7175      +15     
- Misses       7108     7119      +11     

Impacted Files	Coverage Δ
src/os/process_pool.cc	58.16% <24.13%> (-2.48%)	⬇️
src/reactor/base.cc	74.55% <50.00%> (ø)
src/server/port.cc	50.00% <0.00%> (-0.28%)	⬇️
src/os/async_thread.cc	69.73% <0.00%> (+0.87%)	⬆️
src/protocol/base.cc	65.13% <0.00%> (+1.97%)	⬆️
src/os/wait.cc	81.17% <0.00%> (+7.05%)	⬆️

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