Set multiprocessing start method early

[ahopkins]

This PR sets the context as early as possible. Since get_context("spawn") is not the same as set_start_method("spawn") and get_context(), we are adopting the second pattern. This should allow for less "gotchas" when using multiprocessing primitives like multiprocessing.Queue.

[codecov]

Codecov Report

Patch coverage: 78.947% and project coverage change: +0.020 🎉

Comparison is base (4a2b82e) 88.923% compared to head (7e81544) 88.944%.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #2776       +/-   ##
=============================================
+ Coverage   88.923%   88.944%   +0.020%     
=============================================
  Files           92        92               
  Lines         6988      7001       +13     
  Branches      1192      1194        +2     
=============================================
+ Hits          6214      6227       +13     
  Misses         529       529               
  Partials       245       245               

Impacted Files	Coverage Δ
sanic/mixins/startup.py	92.037% <76.470%> (-0.717%)	⬇️
sanic/app.py	90.169% <100.000%> (ø)

... and 1 file with indirect coverage changes

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