[jvm-packages] upgrade to Scala 2.12 (#4574)

* bump scala to 2.12 which requires java 8 and also newer flink and akka

* put scala version in artifactId

* fix appveyor

* fix for scaladoc issue that looks like https://github.com/scala/bug/issues/10509

* fix ci_build

* update versions in generate_pom.py

* fix generate_pom.py

* apache does not have a download for spark 2.4.3 distro using scala 2.12 yet, so for now i use a tgz i put on s3

* Upload spark-2.4.3-bin-scala2.12-hadoop2.7.tgz to our own S3

* Update Dockerfile.jvm_cross

* Update Dockerfile.jvm_cross

jvm-packages/xgboost4j/src/main/scala/ml/dmlc/xgboost4j/scala/rabit/RabitTracker.scala

@@ -131,9 +131,7 @@ private[scala] class RabitTracker(numWorkers: Int, port: Option[Int] = None,
   }
 
   def stop(): Unit = {
-    if (!system.isTerminated) {
-      system.shutdown()
-    }
+    system.terminate()
   }
 
   /**
@@ -170,12 +168,10 @@ private[scala] class RabitTracker(numWorkers: Int, port: Option[Int] = None,
           case Failure(e) =>
             IRabitTracker.TrackerStatus.FAILURE.getStatusCode
         }
-        system.shutdown()
+        system.terminate()
         statusCode
       case Failure(ex: Throwable) =>
-        if (!system.isTerminated) {
-          system.shutdown()
-        }
+        system.terminate()
         IRabitTracker.TrackerStatus.FAILURE.getStatusCode
     }
   }