Sort shuffle read improvements

Hi @sryza , I've done some improvements in current SortSHuffleRead, would you mind help reviewing this and give me some comments, thanks a lot!

1. copy memory shuffle block from off-heap to on-heap if memory is allocated with direct memory to control the off-heap memory size.
2. Some code style modification and better failure processing logic.
Hi @sryza , seems this JIRA addresses the same problem as I mentioned before (https://issues.apache.org/jira/browse/SPARK-4452).