Fix potentially harmful timing issue: _writeInProgress could be observed by BeginSendingMessages to indicate that the loop in SendMessagesFromQueueAsync is still "in flight" while in reality, it has already exited.
Fix potentially harmful timing issue: _writeInProgress could be observed by BeginSendingMessages to indicate that the loop in SendMessagesFromQueueAsync is still "in flight" while in reality, it has already exited.

apache/tinkerpop,robertdale/tinkerpop,robertdale/tinkerpop,apache/incubator-tinkerpop,krlohnes/tinkerpop,robertdale/tinkerpop,apache/tinkerpop,krlohnes/tinkerpop,robertdale/tinkerpop,krlohnes/tinkerpop,krlohnes/tinkerpop,pluradj/incubator-tinkerpop,apache/tinkerpop,krlohnes/tinkerpop,robertdale/tinkerpop,pluradj/incubator-tinkerpop,apache/incubator-tinkerpop,apache/tinkerpop,apache/tinkerpop,apache/incubator-tinkerpop,apache/tinkerpop,apache/tinkerpop,pluradj/incubator-tinkerpop