Sync up with backup target during DR volume activation

Related Issue:

• https://github.com/longhorn/longhorn/issues/5292
• https://github.com/longhorn/longhorn/issues/7945

1. Launch 2 clusters and both have Longhorn installed
2. Set up a backup target.
3. Create a volume and write data in the 1st cluster. Then create 1st backup.
4. Restore the backup as a DR volume in the 2nd cluster.
5. Modify the backup poll interval to a large value.
6. Write more data for the volume in the 1st cluster, and create the 2nd backup.
7. Activate the DR volume in the 2nd cluster. Then verify the data
8. The activated DR volume should contain the latest data.