Refactor impl of Thread::getExitStatus()/Thread::isAlive()

[matyhtf]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 72.22222% with 10 lines in your changes missing coverage. Please review.

Project coverage is 71.16%. Comparing base (d647b78) to head (66b515f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/server/thread.cc	55.55%	8 Missing ⚠️
include/swoole_thread.h	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5695      +/-   ##
==========================================
+ Coverage   71.13%   71.16%   +0.02%     
==========================================
  Files         109      109              
  Lines       16845    16845              
  Branches     3077     3075       -2     
==========================================
+ Hits        11982    11987       +5     
+ Misses       4809     4804       -5     
  Partials       54       54              

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