[BP 7.0.2] - XDCR - throughSeqnoTracker bg scanner may run for a long time

With backfill pipeline, throughSeqnoTrackerSvc implemented a bg scanner to go scan for throughSeqno to make sure the throughSeqno has been processed before stopping the pipeline.

However, if the pipeline is stopped/restarted before this process finishes, the bg scanner will keep not know and will keep running up to a maximum of 5 * 300 sec == 25 minutes.

If a lot of backfill pipeline restart happens, then there will be a lot of these stray bg processes running periodically eating up valuable resources.