SPARK-1786: Edge Partition Serialization 

This appears to address the issue with edge partition serialization.  The solution appears to be just registering the `PrimitiveKeyOpenHashMap`.  However I noticed that we appear to have forked that code in GraphX but retained the same name (which is confusing).  I also renamed our local copy to `GraphXPrimitiveKeyOpenHashMap`.  We should consider dropping that and using the one in Spark if possible.