Introduce scoverage to measure test coverage

### Description

Introduce scoverage to measure coverage. Some small changes have been necessary to exclude generated sources from the report and some blind spots have been filled by adding a couple of tests.
The only caveat is that there is not a command to merge the coverage reports of the individual modules as requested in this [open request](https://github.com/scoverage/sbt-scoverage/issues/4) (open by me, BTW).

### Testing

`sbt -Dconfig.resource=application-ci.conf clean scoverage:test`

### Coverage

 * Server module: 90.78%
 * Client module: 86.85%
 * Common module: 86.73%

This is the highest coverage I've ever seen in a project in which coverage was not being measured...