Typeorm: Use DataSource instead of Connection

[fflorent]

WIP! This PR is open to see whether it passes the tests

Context

Since version 0.3.0, Typeorm functions, methods and classes related to Connections have been deprecated in favor of the use of DataSources.

Proposed solution

Huge refactoration to use DataSource, and remove use of ConnectionManager.

Related issues

Has this been tested?

• [ ] 👍 yes, I added tests to the test suite
• [ ] 💭 no, because this PR is a draft and still needs work
• [x] 🙅 no, because this is not relevant here
• [ ] 🙋 no, because I need help