SPARK-897:  preemptively serialize closures

These commits cause `ClosureCleaner.clean` to attempt to serialize the cleaned closure with the default closure serializer and throw a `SparkException` if doing so fails.  This behavior is enabled by default but can be disabled at individual callsites of `SparkContext.clean`.

Commit 98e01ae8 fixes some no-op assertions in `GraphSuite` that this work exposed; I'm happy to put that in a separate PR if that would be more appropriate.
@willb I think you missed this. Make sure you add comment above this line to explain the reason why we do not check serializable ...