Better shutdown

[jstucke]

• problem: many process in FACT are not properly joined/waited upon after termination
  • this means that the processes are not cleaned up correctly which can lead to zombie processes
  • merges the process termination functions in helper.process into one function
• adds a unified config variable for graceful shutdown timeout
  • for tests, this shutdown time is reduced, which can lead to a significant test speedup
• parallelizes shutdown of backend components and plugin runners

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.63265% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.59%. Comparing base (2c3bdae) to head (e27c271).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/helperFunctions/process.py	77.58%	13 Missing ⚠️
src/start_fact_backend.py	55.55%	4 Missing ⚠️
src/scheduler/analysis/plugin.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1513      +/-   ##
==========================================
- Coverage   93.08%   92.59%   -0.50%     
==========================================
  Files         375      368       -7     
  Lines       23566    21313    -2253     
==========================================
- Hits        21936    19734    -2202     
+ Misses       1630     1579      -51     

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