feat: Graceful shutdown

[kuznetsss]

Fixes #442.

[godexsoft]

Adding some small suggestions. Looks pretty good 👍

[codecov]

Codecov Report

Attention: Patch coverage is 67.29560% with 52 lines in your changes missing coverage. Please review.

Project coverage is 71.63%. Comparing base (12e6fcc) to head (6c52b65).
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	72.41%	4 Missing and 4 partials ⚠️
src/web/ng/impl/ConnectionHandler.cpp	78.37%	1 Missing and 7 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/web/ng/impl/HttpConnection.hpp	33.33%	1 Missing and 1 partial ⚠️
src/app/Stopper.cpp	90.90%	0 Missing and 1 partial ⚠️
src/etl/impl/SourceImpl.hpp	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1801      +/-   ##
===========================================
- Coverage    71.69%   71.63%   -0.06%     
===========================================
  Files          326      330       +4     
  Lines        13307    13413     +106     
  Branches      6761     6826      +65     
===========================================
+ Hits          9540     9608      +68     
- Misses        1913     1921       +8     
- Partials      1854     1884      +30     

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[cindyyan317]

Do we need to consider that shutdown happens during cache loading?

[cindyyan317]

LGTM