Fix discovery server not stopping properly

[ernoaapa]

Previously discovery.Server hang if Stop() were called and never actually close. There were missing make(chan).

[codecov]

Codecov Report

Merging #38 into master will increase coverage by 1.45%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   24.02%   25.48%   +1.45%     
==========================================
  Files          50       50              
  Lines        2439     2433       -6     
==========================================
+ Hits          586      620      +34     
+ Misses       1789     1746      -43     
- Partials       64       67       +3

Impacted Files	Coverage Δ
pkg/discovery/server.go	90.9% <100%> (+90.9%)	⬆️
pkg/discovery/client.go	77.77% <75%> (+77.77%)	⬆️

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