Added better counters to surosinkstat for KafkaSinkV2

These new counters are important for testing and monitoring a production deployment.
@metacret All tests passed on my machine.  However, I do see that there is a race condition at TestQueuedSink:131 that could cause a failure.  To me, if looks like the test is bad.  I think that my change makes the method more correct -- active threads should count as "pending messages" regardless of whether (messagesInQueue == 0) or not.  Please tell me if you'd like me to revert this change.