[SDK] Fix lost wakeup in BatchLogProcessor shutdown/force-flush notify

[denizariyan]

Fixes #4400

Changes

• Fix a notify without lock pattern that leads to lost wake ups leading to stalls. See linked issue for details
• Add stress test to confirm the issue does not occur after the fix

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.48%. Comparing base (bcd4f2d) to head (6812ca2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4518      +/-   ##
==========================================
+ Coverage   83.47%   83.48%   +0.01%     
==========================================
  Files         521      521              
  Lines       20380    20387       +7     
==========================================
+ Hits        17011    17018       +7     
  Misses       3369     3369              

Files with missing lines	Coverage Δ
sdk/src/logs/batch_log_record_processor.cc	87.90% <100.00%> (+0.47%)	⬆️

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