[SPARK-4672][Core]Checkpoint() should clear f to shorten the serialization chain

The related JIRA is https://issues.apache.org/jira/browse/SPARK-4672

The f closure of `PartitionsRDD(ZippedPartitionsRDD2)` contains a `$outer` that references EdgeRDD/VertexRDD, which causes task's serialization chain become very long in iterative GraphX applications. As a result, StackOverflow error will occur. If we set "f = null" in `clearDependencies()`, checkpoint() can cut off the long serialization chain. More details and explanation can be found in the JIRA.