[SPARK-14804][SPARK][GRAPHX] Fix checkpointing of VertexRDD/EdgeRDD

## What changes were proposed in this pull request?

EdgeRDD/VertexRDD overrides checkpoint() and isCheckpointed() to forward these to the internal partitionRDD. So when checkpoint() is called on them, its the partitionRDD that actually gets checkpointed. However since isCheckpointed() also overridden to call partitionRDD.isCheckpointed, EdgeRDD/VertexRDD.isCheckpointed returns true even though this RDD is actually not checkpointed.

This would have been fine except the RDD's internal logic for computing the RDD depends on isCheckpointed(). So for VertexRDD/EdgeRDD, since isCheckpointed is true, when computing Spark tries to read checkpoint data of VertexRDD/EdgeRDD even though they are not actually checkpointed. Through a crazy sequence of call forwarding, it reads checkpoint data of partitionsRDD and tries to cast it to types in Vertex/EdgeRDD. This leads to ClassCastException.

The minimal fix that does not change any public behavior is to modify RDD internal to not use public override-able API for internal logic.
## How was this patch tested?

New unit tests.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #15396 from tdas/SPARK-14804.