[SPARK-18448][CORE] SparkSession should implement java.lang.AutoCloseable like JavaSparkContext

## What changes were proposed in this pull request?

Just adds `close()` + `Closeable` as a synonym for `stop()`. This makes it usable in Java in try-with-resources, as suggested by ash211  (`Closeable` extends `AutoCloseable` BTW)

## How was this patch tested?

Existing tests

Author: Sean Owen <sowen@cloudera.com>

Closes #15932 from srowen/SPARK-18448.

sql/core/src/main/scala/org/apache/spark/sql/SparkSession.scala

@@ -18,6 +18,7 @@
 package org.apache.spark.sql
 
 import java.beans.Introspector
+import java.io.Closeable
 import java.util.concurrent.atomic.AtomicReference
 
 import scala.collection.JavaConverters._
@@ -72,7 +73,7 @@ import org.apache.spark.util.Utils
 class SparkSession private(
     @transient val sparkContext: SparkContext,
     @transient private val existingSharedState: Option[SharedState])
-  extends Serializable with Logging { self =>
+  extends Serializable with Closeable with Logging { self =>
 
   private[sql] def this(sc: SparkContext) {
     this(sc, None)
@@ -647,6 +648,13 @@ class SparkSession private(
     sparkContext.stop()
   }
 
+  /**
+   * Synonym for `stop()`.
+   *
+   * @since 2.2.0
+   */
+  override def close(): Unit = stop()
+
   /**
    * Parses the data type in our internal string representation. The data type string should
    * have the same format as the one generated by `toString` in scala.