Documented connection_pool execution semantics

[anarthal]

Refactored pool internals to remove wait_group
Added an external strand thread-safety test

close #361

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.04%. Comparing base (b7b2061) to head (d4522e0).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #368      +/-   ##
===========================================
- Coverage    99.04%   99.04%   -0.01%     
===========================================
  Files          142      141       -1     
  Lines         7143     7140       -3     
===========================================
- Hits          7075     7072       -3     
  Misses          68       68              

Files with missing lines	Coverage Δ
include/boost/mysql/connection_pool.hpp	100.00% <ø> (ø)
.../impl/internal/connection_pool/connection_node.hpp	98.87% <100.00%> (+0.19%)	⬆️
.../internal/connection_pool/connection_pool_impl.hpp	98.85% <100.00%> (-0.01%)	⬇️

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