WIP: Do not merge! Initial work on using coroutines to simplify some deeply nested async code

[jonesmz]

[codecov[bot]]

Codecov Report

Merging #640 into master will decrease coverage by 20.95%. The diff coverage is 78.84%.

@@             Coverage Diff             @@
##           master     #640       +/-   ##
===========================================
- Coverage   82.68%   61.73%   -20.96%     
===========================================
  Files          46       38        -8     
  Lines        7076     4984     -2092     
===========================================
- Hits         5851     3077     -2774     
- Misses       1225     1907      +682     

[redboltz]

It seems that the CI took over 2 hours 45 minutes at the test phase. So I cancel it. https://github.com/redboltz/mqtt_cpp/runs/876329111?check_suite_focus=true