[STREAMING] [MINOR] Close files correctly when iterator is finished in streaming WAL recovery

Currently there's no chance to close the file correctly after the iteration is finished, change to `CompletionIterator` to avoid resource leakage.

Author: jerryshao <saisai.shao@intel.com>

Closes #6050 from jerryshao/close-file-correctly and squashes the following commits:

52dfaf5 [jerryshao] Close files correctly when iterator is finished

streaming/src/main/scala/org/apache/spark/streaming/util/FileBasedWriteAheadLog.scala

@@ -26,7 +26,7 @@ import scala.language.postfixOps
 import org.apache.hadoop.conf.Configuration
 import org.apache.hadoop.fs.Path
 
-import org.apache.spark.util.ThreadUtils
+import org.apache.spark.util.{CompletionIterator, ThreadUtils}
 import org.apache.spark.{Logging, SparkConf}
 
 /**
@@ -124,7 +124,8 @@ private[streaming] class FileBasedWriteAheadLog(
 
     logFilesToRead.iterator.map { file =>
       logDebug(s"Creating log reader with $file")
-      new FileBasedWriteAheadLogReader(file, hadoopConf)
+      val reader = new FileBasedWriteAheadLogReader(file, hadoopConf)
+      CompletionIterator[ByteBuffer, Iterator[ByteBuffer]](reader, reader.close _)
     } flatMap { x => x }
   }