Cleaner process management

[ahopkins]

This is meant to cleanup the process of shutting the server and processes down in a more controlled manner. It should avoid some of the problems we have seen in the past of sneaky race conditions popping in from time to time in the cleanup process.

Closes #2809

[codecov]

Codecov Report

Attention: 18 lines in your changes are missing coverage. Please review.

Comparison is base (00f2af2) 88.369% compared to head (59bb9c6) 88.191%.

Files	Patch %	Lines
sanic/mixins/startup.py	57.142%	5 Missing and 1 partial ⚠️
sanic/worker/multiplexer.py	28.571%	5 Missing ⚠️
sanic/worker/process.py	58.333%	4 Missing and 1 partial ⚠️
sanic/app.py	50.000%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #2811       +/-   ##
=============================================
- Coverage   88.369%   88.191%   -0.179%     
=============================================
  Files           92        93        +1     
  Lines         7188      7232       +44     
  Branches      1234      1242        +8     
=============================================
+ Hits          6352      6378       +26     
- Misses         581       597       +16     
- Partials       255       257        +2     

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