Fix an issue in ConnectionManager where sendingMessage may create too many unnecessary SendingConnections.
Modified Patch according to comments
Make filter preserve partitioner info, since it can

Conflicts:
	core/src/main/scala/spark/rdd/FilteredRDD.scala
Import small optimization to CoGroupedRDD from master (by Reynold Xin)
Merge branch 'branch-0.6' into imran/0.6-debug