Also, add -Werror to tests so that no warnings in tests are reported anymore.

I fixed those warnings that were easy to fix: redundant imports, unused
variables, etc. (I might have missed some though)

The warnings I deliberately left for now are:
* In `Test.API.UpdateList`: Incomplete pattern matching (we would need
  to think a bit about error handling to fix this)
* In `Test.Utils`: `traceShowId` remains in code (we would need to think
  a bit about proper logging to fix this)

Resolve "Job workers & persistence"

See merge request !371

This reverts commit ff00345c

Maybe this will fix notifications for job finish

It might be that notifications get mixed up when running parallel
test (as they use the same port)

More import refactoring

[TESTED] Worker Tested and README info to create the database, launching and configuring it. Thank your Przemysław for this nice upgrade.

Also, use genericArbitrary in Instances, this simplifies things