[core] Fixed remaining problems with data races

[ethouris]

Fixes:

1. Separated stop() and close() on CChannel. The multiplexer now calls stop() before stopping the queue worker threads, and then calls close() after making sure that these threads have exit. This avoids potential outdated use of the socket descriptor value by the receiver worker thread after another thread has closed it.
2. In the unit test facility, UniqueSocket has been made atomic to avoid data race in the test cases.

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.99%. Comparing base (679f83a) to head (9fc962f).
⚠️ Report is 341 commits behind head on dev.

Files with missing lines	Patch %	Lines
srtcore/api.cpp	50.00%	2 Missing ⚠️
srtcore/channel.cpp	85.71%	1 Missing ⚠️
srtcore/srt_c_api.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              dev    #3381       +/-   ##
===========================================
- Coverage   71.12%   53.99%   -17.13%     
===========================================
  Files         101      115       +14     
  Lines       19776    25337     +5561     
  Branches        0     3896     +3896     
===========================================
- Hits        14065    13681      -384     
- Misses       5711    10237     +4526     
- Partials        0     1419     +1419     

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