Make proactive serializability checking optional.

SparkContext.clean uses ClosureCleaner's proactive serializability
checking by default.  This commit adds a parameter to the clean method
of SparkContext that allows clients to specify that serializability
checking should not occur as part of closure cleaning.
Don't check serializability of DStream transforms.

Since the DStream is reachable from within these closures, they aren't
checkable by the straightforward technique of passing them to the
closure serializer.
Fixed spurious failures in ImplicitOrderingSuite