Better encapsulate diesel's connection to ease the split of editoast_models

[Wadjetz]

Create an abstraction of the diesel database connection and transactions

part of the #6980

How to test this PR:

• Make sure everything work as before
• No deadlocks in tests

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 83.22440% with 77 lines in your changes missing coverage. Please review.

Project coverage is 37.01%. Comparing base (5e3ca65) to head (6b27efa).
Report is 4 commits behind head on dev.

Files with missing lines	Patch %	Lines
editoast/src/views/timetable/stdcm.rs	0.00%	17 Missing ⚠️
editoast/src/views/train_schedule/projection.rs	0.00%	10 Missing ⚠️
editoast/src/views/timetable.rs	0.00%	8 Missing ⚠️
editoast/src/views/train_schedule.rs	68.18%	7 Missing ⚠️
editoast/editoast_models/src/db_connection_pool.rs	91.83%	4 Missing ⚠️
editoast/src/views/infra/mod.rs	84.00%	4 Missing ⚠️
editoast/src/views/rolling_stock.rs	71.42%	4 Missing ⚠️
editoast/src/modelsv2/auth.rs	89.65%	3 Missing ⚠️
editoast/src/views/infra/edition.rs	85.00%	3 Missing ⚠️
editoast/src/views/infra/routes.rs	62.50%	3 Missing ⚠️
... and 10 more

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Additional details and impacted files

@@             Coverage Diff              @@
##                dev    #8712      +/-   ##
============================================
+ Coverage     36.98%   37.01%   +0.03%     
  Complexity     2208     2208              
============================================
  Files          1255     1255              
  Lines        113843   113901      +58     
  Branches       3188     3188              
============================================
+ Hits          42103    42164      +61     
+ Misses        69845    69842       -3     
  Partials       1895     1895              

Flag	Coverage Δ
core	74.73% <ø> (ø)
editoast	72.15% <83.22%> (+0.07%)	⬆️
front	14.97% <ø> (ø)
gateway	2.20% <ø> (ø)
osrdyne	2.71% <ø> (ø)
railjson_generator	87.49% <ø> (ø)
tests	86.37% <ø> (ø)

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[woshilapin]

Big work, will be much better. Thank you 🎉

I request a change on the documentation of the re-implementation of transaction since it's a central but non-trivial function of this new DbConnection. I believe it's important to give as much context as possible to ease future maintenance.

[leovalais]

Huge work! I have a few comments about the API, especially for errors.

That's work for another PR since this one is already massive:

1. Most functions don't require a conn: &mut DbConnection but rather just a conn: DbConnection which became cheap to clone.
2. Maybe we could implement Future for DbConnection so that conn.await <=> conn.write().await.deref_mut()
3. Some sync test util functions might declutter the boilerplate the RwLock introduced.

[leovalais]

Approving in order not to block the PR, but since I made the modifications myself, this doesn't account for much 🤣