Implement graceful shutdown

[szaffarano]

fixes #32

[codecov-commenter]

Codecov Report

Merging #34 (3a58de6) into master (68cc0bf) will decrease coverage by 1.85%.
The diff coverage is 53.19%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
- Coverage   89.22%   87.36%   -1.86%     
==========================================
  Files          10       11       +1     
  Lines         826      863      +37     
==========================================
+ Hits          737      754      +17     
- Misses         51       72      +21     
+ Partials       38       37       -1     

Impacted Files	Coverage Δ
pkg/task/daemon.go	0.00% <0.00%> (ø)
pkg/task/transport/tls.go	96.15% <92.30%> (+1.86%)	⬆️
pkg/task/transport/server.go	100.00% <100.00%> (ø)

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