Connections now support immediate executors

[anarthal]

Immediate completions in connection and any_connection are now correctly dispatched to the token's immediate executor using asio::async_immediate instead of plain asio::post.
Added a section on executors in the reference docs of async functions in connection and any_connection

close #301

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.05%. Comparing base (7ef6ff8) to head (c1b3c05).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #355   +/-   ##
========================================
  Coverage    99.05%   99.05%           
========================================
  Files          143      143           
  Lines         7180     7180           
========================================
  Hits          7112     7112           
  Misses          68       68           

Files with missing lines	Coverage Δ
include/boost/mysql/any_connection.hpp	100.00% <ø> (ø)
include/boost/mysql/connection.hpp	98.90% <ø> (ø)
include/boost/mysql/detail/engine_impl.hpp	97.01% <ø> (ø)

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