[api] remove API 'start' and 'stop'

[wgtdkp]

This PR removes API start and stop. Those two APIs are specific to event loop thread, which should be hide behind the public interface.

[codecov-io]

Codecov Report

Merging #98 into master will decrease coverage by 0.14%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
- Coverage   61.76%   61.61%   -0.15%     
==========================================
  Files          55       55              
  Lines        5408     5393      -15     
==========================================
- Hits         3340     3323      -17     
- Misses       2068     2070       +2     

Impacted Files	Coverage Δ
include/commissioner/commissioner.hpp	43.75% <ø> (ø)
src/app/commissioner_app.cpp	29.43% <ø> (-0.11%)	:arrow_down:
src/library/commissioner_impl.cpp	61.56% <ø> (-0.46%)	:arrow_down:
src/library/commissioner_impl.hpp	100.00% <100.00%> (ø)
src/library/commissioner_safe.cpp	62.24% <100.00%> (ø)
src/library/coap_secure.hpp	83.87% <0.00%> (-0.51%)	:arrow_down:
src/library/udp_proxy.hpp	92.85% <0.00%> (-0.48%)	:arrow_down:
src/library/coap.cpp	71.51% <0.00%> (-0.39%)	:arrow_down: