chore: thread safety annotations on synchronization primitives

[kostasrim]

In order for our mutex classes to work with clang's ThreadSafety analysis (used by ABSL_GUARD_XXX functions) we need to properly annotate them. This PR adds the missing annotations.

• add annotations to SpinLock
• add annotations to Mutex
• add annotations to SharedMutex

// TODO

• turn on -Wthread-safety
• fix any missing annotations

[kostasrim]

@romange this is not ready, I will ping

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.85%. Comparing base (d30de86) to head (7b6abdb). Report is 85 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #305 +/- ## ========================================== + Coverage 77.60% 77.85% +0.24% ========================================== Files 103 108 +5 Lines 7824 8101 +277 ========================================== + Hits 6072 6307 +235 - Misses 1752 1794 +42 ```

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