Posgresql for framework development

[jakubkoci]

As a developer, I want to run all the tests with the least amount of external dependencies, so that I can contribute to a small part of the framework without bothering about the rest so much and not breaking it.

Current:

• There is a test postgres.test.ts requiring the installation of PostgreSQL database (if I understand it correctly).

Expected:

• We need to add a description of how to install Postgres dependencies to DEVREADME.md or we need to figure out how to get rid of it by splitting tests into some separate groups.

[TimoGlastra]

Related issue: #758