Ensure assertions in Graph.apply are asserted.

The Graph.apply test in GraphSuite had some assertions in a closure in
a graph transformation. This caused two problems:

1. because assert() was called, test classes were reachable from the
closures, which made them not serializable, and

2. (more importantly) these assertions never actually executed, since
they occurred within a lazy map()

This commit simply changes the Graph.apply test to collects the graph
triplets so it can assert about each triplet from a map method.
Make proactive serializability checking optional.

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