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

[codecov[bot]]

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 :warning:
include/swoole_thread.h	75.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ 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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