[SPARK-5831][Streaming]When checkpoint file size is bigger than 10, then delete the old ones

Author: Xutingjun <1039320815@qq.com>

Closes #4621 from XuTingjun/checkpointFile and squashes the following commits:

b5f2816 [Xutingjun] fix bug

streaming/src/main/scala/org/apache/spark/streaming/Checkpoint.scala

@@ -152,7 +152,7 @@ class CheckpointWriter(
 
           // Delete old checkpoint files
           val allCheckpointFiles = Checkpoint.getCheckpointFiles(checkpointDir, fs)
-          if (allCheckpointFiles.size > 4) {
+          if (allCheckpointFiles.size > 10) {
             allCheckpointFiles.take(allCheckpointFiles.size - 10).foreach(file => {
               logInfo("Deleting " + file)
               fs.delete(file, true)