[cli] fix signal handling

[wgtdkp]

Synchronization primitives (std::mutex, std::future) in namespace std are used to synchronize calls to the commissioner API. Unfortunately, those synchronization primitives are not signal-safe and will result in deadlock.

This PR fixes this by using a dedicated thread to wait for the signal.

[codecov-commenter]

Codecov Report

Merging #138 into master will increase coverage by 0.05%. The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
+ Coverage   64.77%   64.82%   +0.05%     
==========================================
  Files          52       52              
  Lines        4749     4754       +5     
==========================================
+ Hits         3076     3082       +6     
+ Misses       1673     1672       -1     

Impacted Files	Coverage Δ
src/app/cli/main.cpp	93.75% <90.90%> (+4.86%)	:arrow_up:

[jwhui]

@wgtdkp , please help resolve conflicts.

[wgtdkp]

@jwhui done.