core: multithread core workers

[eckter]

Fix #9572

Configurable with a WORKER_THREADS=42 env variable, with a default set to the number of processors.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 27 lines in your changes missing coverage. Please review.

Project coverage is 42.49%. Comparing base (f05eecf) to head (f228935).
Report is 2 commits behind head on dev.

Files with missing lines	Patch %	Lines
...re/src/main/java/fr/sncf/osrd/cli/WorkerCommand.kt	0.00%	27 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #9591      +/-   ##
============================================
- Coverage     42.50%   42.49%   -0.02%     
  Complexity     2272     2272              
============================================
  Files          1312     1312              
  Lines        105588   105608      +20     
  Branches       3304     3305       +1     
============================================
- Hits          44881    44873       -8     
- Misses        58754    58782      +28     
  Partials       1953     1953              

Flag	Coverage Δ
core	74.95% <0.00%> (-0.11%)	⬇️
editoast	73.70% <ø> (-0.03%)	⬇️
front	18.79% <ø> (ø)
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	86.74% <ø> (ø)

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[Khoyo]

Does this do parallel consume? Or does this launch a single task on the threadpool (and we would need to actually call consume multiple times on separate channels?)

[eckter]

Does this do parallel consume? Or does this launch a single task on the threadpool (and we would need to actually call consume multiple times on separate channels?)

The "consume" is only done on a single thread, but the processing itself is dispatched to the executor.

The alternative would be to create several channels and call several consumes, but I struggled to make it work and to have it actually work in parallel. (But I can try again later if the current solution bothers anyone)