Replace sqlite with embedded Postgres

[oxisto]

Instead of having basically two database systems sqlite and Postgres, we could possibly get rid of sqlite with https://github.com/fergusstrange/embedded-postgres.

In the past we sometimes had issues that unit tests and the "real" application would behave differently because one is running with sqlite, the other with a real Postgres DB.

We basically have two use cases for sqlite/embedded Postgres:

• Unit tests of our services that need persistence. Not all of the unit tests need that, so we probably would need some kind of util function to "enable" it for a particular test file. I would imagine that embedded Postgres is a little bit slower than sqlite
• The "demo" mode. In this mode we are currently using an in-memory sqlite database, just to demonstrate things, such as showing the UI, etc. This could easily be done with an embedded Postgres

Con:

• Not sure about the file size?
• Speed?

Pro:

• We probably could get rid of a few dependencies, since our non-CGO sqlite dependencies includes a lot of transitive dependencies that basically include the whole C standard library transcoded into Go, such as modernc.org/libc