[SPARK-4301] StreamingContext should not allow start() to be called after calling stop()

In Spark 1.0.0+, calling `stop()` on a StreamingContext that has not been started is a no-op which has no side-effects. This allows users to call `stop()` on a fresh StreamingContext followed by `start()`. I believe that this almost always indicates an error and is not behavior that we should support. Since we don't allow `start() stop() start()` then I don't think it makes sense to allow `stop() start()`.

The current behavior can lead to resource leaks when StreamingContext constructs its own SparkContext: if I call `stop(stopSparkContext=True)`, then I expect StreamingContext's underlying SparkContext to be stopped irrespective of whether the StreamingContext has been started. This is useful when writing unit test fixtures.

Prior discussions:
- https://github.com/apache/spark/pull/3053#discussion-diff-19710333R490
- https://github.com/apache/spark/pull/3121#issuecomment-61927353
Why is the nested if necessary? Why not
 if (state == Stopped) { ... } else if (state == Initialized) { ... } else { ... }