[SPARK-12001] Allow partially-stopped StreamingContext to be completely stopped

If `StreamingContext.stop()` is interrupted midway through the call, the context will be marked as stopped but certain state will have not been cleaned up. Because `state = STOPPED` will be set, subsequent `stop()` calls will be unable to finish stopping the context, preventing any new StreamingContexts from being created.

This patch addresses this issue by only marking the context as `STOPPED` once the `stop()` has successfully completed which allows `stop()` to be called a second time in order to finish stopping the context in case the original `stop()` call was interrupted.

I discovered this issue by examining logs from a failed Jenkins run in which this race condition occurred in `FailureSuite`, leaking an unstoppable context and causing all subsequent tests to fail.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #9982 from JoshRosen/SPARK-12001.