[SPARK-4183] Close transport-related resources between SparkContexts

A leak of event loops may be causing test failures.
I propose, as part of this PR, to make changes only to the streaming unit test to fix this. You could change the test to create a spark context first and then create a streaming context. Then you will have an handler to the sparkcontext and explicitly stop it. 

Alternatively you could create the streamigncontext as it is, and do `sc = ssc.sparkContext` to get a handle on the spark context created by the streamingcontext