Fix handling interruption of blocked WALSource

The getUnackedSource call in NaiveFileWALManager blocks if there are no elements to process in queue. If interrupted and the logged and sent queues are empty, this should exit cleanly. If both logged and sent queues are empty we can definitely exit with without error.

If the logged queue still contains data, and this is interrupted, it should definitely fail.

It is debatable if we should exit cleanly if the logged queue is empty and the sent queue is not (sent but not acked). This patch exits cleanly only if the ack has been received as well.