Scheduler won't resume if lightning exits abnormally

[rleungx]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. start a cluster with TiUP
2. use lightning to import data
3. kill lightning with kill -9

2. What did you expect to see? (Required)

The schedulers are resumed after 5 minutes.

3. What did you see instead (Required)

The schedulers are paused after 5 minutes.

4. What is your TiDB version? (Required)

master