Minor improvements of AppState management in editoast

[leovalais]

• editoast: deprecate DbPool V1 and remove it from the AppState
• editoast: destructure AppState early in handlers for consistency
• editoast: fix minor destructuration inconsistencies

TIP
Rapidly reviewable commit by commit

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 72.83951% with 44 lines in your changes missing coverage. Please review.

Project coverage is 79.83%. Comparing base (02ccfc8) to head (fd2a3d9).
Report is 9 commits behind head on dev.

Files with missing lines	Patch %	Lines
editoast/src/views/timetable.rs	38.88%	11 Missing ⚠️
editoast/src/views/train_schedule/projection.rs	0.00%	8 Missing ⚠️
editoast/src/views/infra/routes.rs	46.15%	7 Missing ⚠️
editoast/src/views/train_schedule.rs	82.85%	6 Missing ⚠️
editoast/src/views/infra/pathfinding.rs	0.00%	5 Missing ⚠️
editoast/src/views/mod.rs	63.63%	4 Missing ⚠️
editoast/src/views/infra/edition.rs	50.00%	1 Missing ⚠️
editoast/src/views/layers.rs	0.00%	1 Missing ⚠️
editoast/src/views/path/properties.rs	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##              dev    #9935       +/-   ##
===========================================
+ Coverage   38.21%   79.83%   +41.61%     
===========================================
  Files         992     1048       +56     
  Lines       92158   105057    +12899     
  Branches     1186      756      -430     
===========================================
+ Hits        35221    83874    +48653     
+ Misses      56486    21142    -35344     
+ Partials      451       41      -410     

Flag	Coverage Δ
editoast	73.36% <72.83%> (-0.04%)	⬇️
front	89.33% <ø> (+69.14%)	⬆️
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	87.00% <ø> (ø)

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[leovalais]

Thanks @woshilapin for the review! I'll let you resolve the conversations if the changes are ok with you. (I implemented FromRef for Arc<CoreClient> instead as it's clone isn't as cheap as cloning an Arc.)