[mdns] move mDNS out of the commissioner library

[wgtdkp]

This PR moves discovery with mDNS out of the Commissioner library:

• It is better for the mobile App/OS to use native mDNS service rather than the one provided by this library.
• We still keep the borderagent discover command for the CLI App.

address issue: #51.
related issue: #31.

[codecov-io]

Codecov Report

Merging #53 into master will increase coverage by 0.02%.
The diff coverage is 57.31%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   60.32%   60.35%   +0.02%     
==========================================
  Files          52       54       +2     
  Lines        5379     5277     -102     
==========================================
- Hits         3245     3185      -60     
+ Misses       2134     2092      -42     

Impacted Files	Coverage Δ
include/commissioner/commissioner.hpp	100.00% <ø> (ø)
src/app/border_agent.hpp	100.00% <ø> (ø)
src/app/cli/interpreter.hpp	100.00% <ø> (ø)
src/app/commissioner_app.hpp	75.00% <ø> (ø)
src/app/file_logger.hpp	0.00% <0.00%> (ø)
src/app/json.hpp	0.00% <ø> (ø)
src/library/commissioner_impl.hpp	100.00% <ø> (ø)
src/library/commissioner_safe.cpp	62.20% <ø> (-1.57%)	⬇️
src/app/file_util.cpp	28.20% <28.20%> (ø)
src/app/border_agent.cpp	52.67% <52.67%> (ø)
... and 12 more