fix: race condition in Uni.combine().all()

[jponge]

There was a race condition in UniAndCombination due to concurrent threads calling AndSupervisor::check, but the item and failure fields in UniHandler were not volatile, hence the count of incomplete inner results could be wrong with no further signal to make another check.

An easy fix was to make them volatile, but a work-in-progress guard is more efficient, and cleaner.

Fixes #1993

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.17%. Comparing base (bcec086) to head (0ef8ca6).
⚠️ Report is 30 commits behind head on main.

Files with missing lines	Patch %	Lines
...allrye/mutiny/operators/uni/UniAndCombination.java	87.50%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2014      +/-   ##
============================================
+ Coverage     89.14%   89.17%   +0.02%     
- Complexity     3068     3069       +1     
============================================
  Files           409      409              
  Lines         13100    13105       +5     
  Branches       1652     1654       +2     
============================================
+ Hits          11678    11686       +8     
+ Misses          807      800       -7     
- Partials        615      619       +4     

Files with missing lines	Coverage Δ
...allrye/mutiny/operators/uni/UniAndCombination.java	92.52% <87.50%> (+1.34%)	⬆️

... and 13 files with indirect coverage changes

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