enable more warnings
enable more warnings

* and add -Werror flag, disallowing warnings
* bump maven-plugin-api version to avoid the evil warning
  "Supported source version 'RELEASE_6' from annotation processor 'org.eclipse.sisu.space.SisuIndexAPT6' less than -source '1.8'"
* explicitly add guava dependency
* note: -Xlint:processing would warn about
  org.apache.maven.plugins.annotations.* not being "claimed",
  this is not a problem it seems.

vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa