Per-operation cancellation for connection_pool

[anarthal]

close #197

[codecov]

Codecov Report

Attention: Patch coverage is 98.21429% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.02%. Comparing base (0f843cd) to head (e072e3f).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
.../internal/connection_pool/connection_pool_impl.hpp	98.14%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #348      +/-   ##
===========================================
- Coverage    99.03%   99.02%   -0.01%     
===========================================
  Files          144      144              
  Lines         7148     7192      +44     
===========================================
+ Hits          7079     7122      +43     
- Misses          69       70       +1     

Files with missing lines	Coverage Δ
include/boost/mysql/connection_pool.hpp	100.00% <ø> (ø)
...t/mysql/impl/internal/ssl_context_with_default.hpp	100.00% <100.00%> (ø)
.../internal/connection_pool/connection_pool_impl.hpp	97.89% <98.14%> (-0.01%)	⬇️

... and 1 file with indirect coverage changes

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