Source tasks are no longer stopped on a separate thread

Before https://github.com/apache/kafka/pull/9669, in distributed mode, the SourceTask::stop method would be invoked on the herder tick thread, which is a separate thread from the dedicated thread which was responsible for polling data from the task and producing it to Kafka.

This aligned with the Javadocs for SourceTask:poll, which state:

The task will be stopped on a separate thread, and when that happens this method is expected to unblock, quickly finish up any remaining processing, and return.

However, it came with the downside that the herder's tick thread would be blocked until the invocation of SourceTask::stop completed, which could result in major parts of the worker's REST API becoming unavailable and even the worker falling out of the cluster.

As a result, in https://github.com/apache/kafka/pull/9669, we changed the logic for task shutdown to cause SourceTask::stop to be invoked on the dedicated thread for the task (i.e., the one responsible for polling data from it and producing that data to Kafka).

This altered the semantics for SourceTask:poll and SourceTask::stop and may have broken connectors that block during poll with the expectation that stop can and will be invoked concurrently as a signal that any ongoing polls should be interrupted immediately.

Although reverting the fix is likely not a viable option (blocking the herder thread on interactions with user-written plugins is high-risk and we have tried to eliminate all instances of this where feasible), we may try to restore the expected contract by spinning up a separate thread exclusively for invoking SourceTask::stop separately from the dedicated thread for the task and the herder's thread.