* Enable Java 11 features
As of this commit Java 11 must be used to build. The generated bytecode
is still at Java 8 due to App Engine task queue limit.
Also fixed a bug where the included google-java-format jar file is not
used, requiring the user to install it separately.
See: https://cloud.google.com/appengine/docs/standard/java/taskqueue/push
* Get rid of all remaining JUnit 4 usages except in prober & proxy subprojects
Caveat: Test suites aren't yet implemented in JUnit 5 so we still use the ones
from JUnit 5 in the core subproject.
* Fix some build errors
