newSingleThread/ThreadPoolContext shall not include a Job

[elizarov]

The coroutine context that is returned by newSingleThread/newThreadPoolContext function should not include a Job. It should provide only an instance of CoroutineDispatcher similarly to what Executor.asCoroutineDispatcher() extension does.

As Job in this context causes problems when multiple contexts are combined, as it overrides the Job from the parent context, blocking the propagation of cancellation.