editoast: fix db connections pool size

[flomonster]

By default:

• diesel create a pool of nb_worker * 4 connections
• rocket instantiates threads * 2 worker

On my machine, I had 64 connections created.

This is an issue cause we can easily reach postgresql limit (around 200).
I suspect that this is the cause of issues within the github CI

[Khoyo]

LGTM

[codecov]

Codecov Report

Merging #2663 (1f25d30) into dev (76182fe) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##                dev    #2663   +/-   ##
=========================================
  Coverage     37.29%   37.30%           
  Complexity     1679     1679           
=========================================
  Files           578      578           
  Lines         17598    17599    +1     
  Branches       2435     2435           
=========================================
+ Hits           6564     6566    +2     
+ Misses        10657    10656    -1     
  Partials        377      377           

Flag	Coverage Δ
editoast	70.82% <100.00%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
editoast/src/client/postgres_config.rs	50.00% <ø> (ø)
editoast/src/schema/signal.rs	78.57% <ø> (ø)
editoast/src/views/infra/mod.rs	86.95% <ø> (ø)
editoast/src/views/infra/objects.rs	100.00% <ø> (ø)
editoast/src/views/mod.rs	100.00% <ø> (ø)
editoast/src/main.rs	30.58% <100.00%> (+0.82%)	⬆️
editoast/src/generated_data/utils.rs	84.21% <0.00%> (+5.26%)	⬆️

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