* Upgrade error-prone to 3.3.4
This would fix the failure with openjdk 11.0.9 in
3.3.3.
Fixed new antipatterns raised by the new version:
- Replaced unnecessary lambdas with methods.
- Switched wait/sleep calls to equivalent methods using java.time types
- Types inheriting Object.toString() should not be assigned to string
parameter in logging statements.
* 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
