StreamingQuery.stop() should not block indefinitely

The stop() method on a Streaming Query awaits the termination of the stream execution thread. However, the stream execution thread may block forever depending on the streaming source implementation (like in Kafka, which runs UninterruptibleThreads).

This causes control flow applications to hang indefinitely as well. We'd like to introduce a timeout to stop the execution thread, so that the control flow thread can decide to do an action if a timeout is hit.