fixInfiniteLoopIssueWhenSourceTaskGotCancelled

[xinlian12]

Issue:
Post this change in kafka connect framework the connector uses a single thread for both
poll() as well as for stop(). Before that change,
the SourceTask::stop method would be invoked on a separate thread from
the one that did the actual data processing for the task (polling the task
for records, transforming and converting those records, then sending them
to Kafka). After that change, SourceTask::stop() is being invoked on the
same thread that handled data processing for the task. This had the effect
that tasks which blocked indefinitely in the SourceTask::poll method with the expectation
that they could stop blocking when SourceTask::stop
was invoked would no longer be capable of graceful shutdown, and may even
hang forever and the reverse is also true. Which is if stop() sleeps on the thread forever
then poll() would never be able to continue.

This issue also related to #543

Changes:

1. Remove the empty check from the stop() method
2. Adding .block when calling changeFeedProcessor.stop(), else the changeFeedProcessor will not be stopped and all internal tasks are still running
3. using dedicated Kafka-cfp-bounded-elastic schedulers

[kushagraThapar]

LGTM, thanks @xinlian12