TTL should be able to tolerate region disappearance

[YangKeao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Create a table with millions empty regions.
2. Start a TTL job for it.

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

TTL job starts successfully.

3. What did you see instead (Required)

TTL job failed to start because split scan ranges: region not found for regionID.

4. What is your TiDB version? (Required)