[SPARK-23129][CORE] Make deserializeStream of DiskMapIterator init lazily

## What changes were proposed in this pull request?

Currently,the deserializeStream in ExternalAppendOnlyMap#DiskMapIterator init when DiskMapIterator instance created.This will cause memory use overhead when ExternalAppendOnlyMap spill too much times.

We can avoid this by making deserializeStream init when it is used the first time.
This patch make deserializeStream init lazily.

## How was this patch tested?

Exist tests

Author: zhoukang <[email protected]>

Closes #20292 from caneGuy/zhoukang/lay-diskmapiterator.

Author: caneGuy

core/src/main/scala/org/apache/spark/util/collection/ExternalAppendOnlyMap.scala

@@ -463,7 +463,7 @@ class ExternalAppendOnlyMap[K, V, C](
 
     // An intermediate stream that reads from exactly one batch
     // This guards against pre-fetching and other arbitrary behavior of higher level streams
-    private var deserializeStream = nextBatchStream()
+    private var deserializeStream: DeserializationStream = null
     private var nextItem: (K, C) = null
     private var objectsRead = 0
 
@@ -528,27 +528,30 @@ class ExternalAppendOnlyMap[K, V, C](
     override def hasNext: Boolean = {
       if (nextItem == null) {
         if (deserializeStream == null) {
-          return false
+          // In case of deserializeStream has not been initialized
+          deserializeStream = nextBatchStream()
+          if (deserializeStream == null) {
+            return false
+          }
         }
         nextItem = readNextItem()
       }
       nextItem != null
     }
 
     override def next(): (K, C) = {
-      val item = if (nextItem == null) readNextItem() else nextItem
-      if (item == null) {
+      if (!hasNext) {
         throw new NoSuchElementException
       }
+      val item = nextItem
       nextItem = null
       item
     }
 
     private def cleanup() {
       batchIndex = batchOffsets.length  // Prevent reading any other batch
-      val ds = deserializeStream
-      if (ds != null) {
-        ds.close()
+      if (deserializeStream != null) {
+        deserializeStream.close()
         deserializeStream = null
       }
       if (fileStream != null) {