Reproducible Builds

A long term goal for reliability is reproducible builds.

In order to do this we have to make the maven build process and the native compiling reproducible.

For maven, see https://maven.apache.org/guides/mini/guide-reproducible-builds.html:
* fix all plugin versions (including default plugins such as maven-clean-plugin)
* add project.build.outputTimestamp (can be an arbitrary but fixed date such as 2000-01-01)
* verify if two builds yield identical artifacts:
```
./mvnw clean install
./mvnw clean package artifact:compare -DskipTests
```

For native compilation:
tbd

Start with the Maven reproducible-builds guide linked in the issue, then run ./mvnw clean install and ./mvnw clean package artifact:compare -DskipTests to establish the current result. Done means Maven artifacts are reproducible after fixing plugin versions and the output timestamp; native compilation is still marked TBD.

Written by the indexing model from the issue text.