refactor: use sequelize-pool for pooling

[sushantdhiman]

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• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
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Description of change

This PR will migrate Sequelize to https://github.com/sushantdhiman/sequelize-pool which is a modernized fork of generic-pool@2.5

In theory this should solve issues like

• Connection pool doesnt reconnect with MySQL after server dies and comes back (only with replication configured) #9541
• Connections are not automatically closed #8468

[codecov]

Codecov Report

Merging #10051 into master will increase coverage by 0.04%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master   #10051      +/-   ##
==========================================
+ Coverage    96.3%   96.35%   +0.04%     
==========================================
  Files          63       63              
  Lines        9412     9399      -13     
==========================================
- Hits         9064     9056       -8     
+ Misses        348      343       -5

Impacted Files	Coverage Δ
lib/sequelize.js	96.62% <ø> (ø)	⬆️
lib/dialects/mssql/connection-manager.js	88.6% <100%> (ø)	⬆️
lib/dialects/postgres/connection-manager.js	93.8% <100%> (+0.16%)	⬆️
lib/dialects/mysql/connection-manager.js	96.55% <100%> (+1.31%)	⬆️
lib/dialects/abstract/connection-manager.js	90% <87.5%> (+2.39%)	⬆️

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[SystemDisc]

Is there any way this change can be implemented in v4? We're currently experiencing occasional issues with RequestResource timed out being thrown in generic-pool when attempting to run a query like modelInstance.find(...). We believe this change would resolve the issue we're having, but we cannot use beta software.

If this change cannot be implemented in v4. Is there any estimate on when v5 might come out of beta?

Thank you for your time.

[SimonSchick]

So that's what that is, we experienced the same issue.
@sushantdhiman I think it's fair to roll out v5 at this point, it has enough changes and fixes as is.

[kelvintaywl]

that would be really great! 🙇
we are just about to deliberate on using v5 beta for a fix.