Faster PR validation

This reworks the build so we can speed up PR validation, publishing Scala to maven once, and consuming that downstream in the other jobs (by maven/sbt/dbuild)

We're standardizing on putting all our java properties in versions.properties and passing that around between the jenkins pr validation jobs, having them consumed by the scripts and dbuild. This way we can keep all module versions in synch and don't need to change infrastructure (much) when new properties are needed.