improve mqtt QoS and return types

[maurerle]

The return parameter of send_message was used inconsistently.
This is fixed through this PR.

fixes #31

I benchmarked the code a little with the code given in #142 for different QoS and transport methods.
Using the default TCP with default QoS of 0 seems to be the desirable choice.

Note that the QoS of the subscribe side, is used as a maximum QoS supported.

It is desired to use set_tcp_nodelay=true for MQTT brokers to reduce the roundtrip time as noted in #142

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.97%. Comparing base (037f0ec) to head (373ad84).
Report is 3 commits behind head on development.

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #143      +/-   ##
===============================================
- Coverage        88.97%   88.97%   -0.01%     
===============================================
  Files               22       22              
  Lines             2459     2458       -1     
===============================================
- Hits              2188     2187       -1     
  Misses             271      271              

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