feat: Graceful shutdown

[kuznetsss]

For #442. This is a WIP PR containing current state of graceful shutdown.

[godexsoft]

Adding some small suggestions. Looks pretty good 👍

[kuznetsss]

This should be covered by test.

[codecov]

Codecov Report

Attention: Patch coverage is 69.71831% with 43 lines in your changes missing coverage. Please review.

Project coverage is 71.42%. Comparing base (590c07a) to head (baa6727).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/app/Stopper.hpp	10.52%	0 Missing and 17 partials ⚠️
src/etl/impl/SubscriptionSource.cpp	73.33%	4 Missing and 4 partials ⚠️
src/etl/ETLService.hpp	12.50%	7 Missing ⚠️
src/web/ng/Server.cpp	72.22%	4 Missing and 1 partial ⚠️
src/app/ClioApplication.cpp	0.00%	3 Missing ⚠️
src/app/Stopper.cpp	90.90%	0 Missing and 1 partial ⚠️
src/etl/impl/SourceImpl.hpp	66.66%	0 Missing and 1 partial ⚠️
src/web/ng/impl/ConnectionHandler.cpp	96.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1801      +/-   ##
===========================================
- Coverage    71.45%   71.42%   -0.03%     
===========================================
  Files          321      325       +4     
  Lines        13227    13321      +94     
  Branches      6737     6789      +52     
===========================================
+ Hits          9451     9515      +64     
- Misses        1958     1966       +8     
- Partials      1818     1840      +22     

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