Prohibit concurrent join

[matyhtf]

No description provided.

[codecov]

Codecov Report

Merging #4442 (3e27c11) into master (af1359c) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4442      +/-   ##
==========================================
+ Coverage   52.37%   52.39%   +0.01%     
==========================================
  Files          73       73              
  Lines       14882    14882              
==========================================
+ Hits         7795     7797       +2     
+ Misses       7087     7085       -2     

Impacted Files	Coverage Δ
src/os/async_thread.cc	72.10% <0.00%> (+0.85%)	⬆️

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