Fixed a race condition in per-operation cancellation in variant_stream

[anarthal]

close #454

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.02%. Comparing base (e988c9a) to head (444ca7f).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #457   +/-   ##
========================================
  Coverage    99.02%   99.02%           
========================================
  Files          142      142           
  Lines         7260     7262    +2     
========================================
+ Hits          7189     7191    +2     
  Misses          71       71           

Files with missing lines	Coverage Δ
...clude/boost/mysql/impl/internal/variant_stream.hpp	100.00% <100.00%> (ø)

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