TTL scan task will leak when down scaling the scan workers if it failed to cancel the task.

[YangKeao]

Bug Report

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If the scan worker failed to cancel the task, the task.result will always be nil, and it'll never be regarded as finished. See the following function:

func (m *taskManager) resizeScanWorkers(count int) error {
	var err error
	var canceledWorkers []worker
	m.scanWorkers, canceledWorkers, err = m.resizeWorkers(m.scanWorkers, count, func() worker {
		return newScanWorker(m.delCh, m.notifyStateCh, m.sessPool)
	})
	for _, w := range canceledWorkers {
		...

		result := s.PollTaskResult()
		if result != nil {
			jobID = result.task.JobID
			scanID = result.task.ScanID

			scanErr = result.err
		} else {
			// if the scan worker failed to poll the task, it's possible that the `WaitStopped` has timeout
			// we still consider the scan task as finished
			curTask := s.CurrentTask()
			if curTask == nil {
				continue
			}
			jobID = curTask.JobID
			scanID = curTask.ScanID
			scanErr = errors.New("timeout to cancel scan task")
		}

...

		task.result = result
	}
	return err
}

It's easy to fix. We need to assign a result to the scan task.