SITUATION: ShuffledDStream run tasks whether dstream has partition items  or not
FIX: ShuffledDStream run tasks only when dstream has partition items
DStream run tasks only when dstream has partition items
DStream run tasks only when dstream has partition items
DStream run tasks only when dstream has partition items
Merge branch 'sql-memory-patch' of https://github.com/guowei2/spark into sql-memory-patch