SPARK-4567. Make SparkJobInfo and SparkStageInfo serializable

Author: Sandy Ryza <sandy@cloudera.com>

Closes #3426 from sryza/sandy-spark-4567 and squashes the following commits:

cb4b8d2 [Sandy Ryza] SPARK-4567. Make SparkJobInfo and SparkStageInfo serializable

(cherry picked from commit 5e4c06f)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>

core/src/main/java/org/apache/spark/SparkJobInfo.java

@@ -17,13 +17,15 @@
 
 package org.apache.spark;
 
+import java.io.Serializable;
+
 /**
  * Exposes information about Spark Jobs.
  *
  * This interface is not designed to be implemented outside of Spark.  We may add additional methods
  * which may break binary compatibility with outside implementations.
  */
-public interface SparkJobInfo {
+public interface SparkJobInfo extends Serializable {
   int jobId();
   int[] stageIds();
   JobExecutionStatus status();

core/src/main/java/org/apache/spark/SparkStageInfo.java

@@ -17,13 +17,15 @@
 
 package org.apache.spark;
 
+import java.io.Serializable;
+
 /**
  * Exposes information about Spark Stages.
  *
  * This interface is not designed to be implemented outside of Spark.  We may add additional methods
  * which may break binary compatibility with outside implementations.
  */
-public interface SparkStageInfo {
+public interface SparkStageInfo extends Serializable {
   int stageId();
   int currentAttemptId();
   long submissionTime();