connection_pool destructor now cancels async operations

[anarthal]

Improved connection_pool docs on thread-safety.

close #370

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.04%. Comparing base (6842c2d) to head (a6fe66d).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #372   +/-   ##
========================================
  Coverage    99.04%   99.04%           
========================================
  Files          143      143           
  Lines         7154     7157    +3     
========================================
+ Hits          7086     7089    +3     
  Misses          68       68           

Files with missing lines	Coverage Δ
include/boost/mysql/connection_pool.hpp	100.00% <100.00%> (ø)

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