[app] clear network data when stopped

[wgtdkp]

This PR clears data associated with the connected Thread Network when the commissioner is stopped (disconnected from the Thread Network).

[codecov-io]

Codecov Report

Merging #86 into master will increase coverage by 0.14%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   60.12%   60.27%   +0.14%     
==========================================
  Files          54       55       +1     
  Lines        5317     5334      +17     
==========================================
+ Hits         3197     3215      +18     
+ Misses       2120     2119       -1     

Impacted Files	Coverage Δ
src/app/commissioner_app.hpp	75.00% <ø> (ø)
src/app/commissioner_app.cpp	28.33% <100.00%> (+0.76%)	:arrow_up:
src/library/socket.hpp	66.66% <0.00%> (-8.34%)	:arrow_down:
src/library/commissioning_session.hpp	62.50% <0.00%> (-4.17%)	:arrow_down:
src/app/file_logger.cpp	65.62% <0.00%> (-2.03%)	:arrow_down:
src/library/commissioning_session.cpp	85.96% <0.00%> (-1.80%)	:arrow_down:
src/library/dtls.cpp	81.12% <0.00%> (-1.15%)	:arrow_down:
src/library/coap.cpp	71.89% <0.00%> (-0.09%)	:arrow_down:
src/library/dtls.hpp	66.66% <0.00%> (ø)
src/library/endpoint.hpp	100.00% <0.00%> (ø)
... and 8 more