Add constexpr keyword to lots of small functions where possible

[jonesmz]

No description provided.

[codecov-io]

Codecov Report

Merging #451 into master will decrease coverage by <.01%.
The diff coverage is 82.81%.

@@            Coverage Diff             @@
##           master     #451      +/-   ##
==========================================
- Coverage   84.94%   84.93%   -0.01%     
==========================================
  Files          40       40              
  Lines        6342     6341       -1     
==========================================
- Hits         5387     5386       -1     
  Misses        955      955

[jonesmz]

It's very unclear to me why the code coverage would have changed due to this PR.

[redboltz]

I re-run the bottom 4 tasks on travis-ci. They send codecov reports.
Sometimes some of them seems to fail to send report and it is NOT logged.

Perhaps codecov has time limit from the first report receiving to the last report receiving.

To be honest, I don't read much the codecov document. I just modify some example.

Here is codecov report code:
https://github.com/redboltz/mqtt_cpp/blob/master/.travis.yml#L102-L107

I guess that we can improve it.

[redboltz]

Recovered by re-run bottom four builds. - Coverage 84.94% 84.93% -0.01%

[jonesmz]

Still weird that the coverage decreased even though I removed an unused function. Oh well.

[jonesmz]

Either way, I think this code is ready to merge.

Let me know if there's anything you want me to change.

[redboltz]

Looks good to me. Thanks.