Incremental backup / diff scan does not handle rows that are inserted and deleted between two incrementals correctly

I did the following sequence of operations:

1. Insert 100 million rows
2. Update 1 out of every 11 rows
3. Make a full backup
4. Insert 100 million more rows, after the original rows in keyspace
5. Delete 1 out of every 23 rows
6. Make an incremental backup

Restore failed to apply the incremental backup, failing with an error like

java.lang.RuntimeException: failed to write 1000 rows from DataFrame to Kudu; sample errors:

Due to another bug, there's no sample errors, but after hacking around that bug, I found that the incremental contained a row with a DELETE action for a key that is not present in the full backup. That's because the row was inserted in step 4 and deleted in step 5, between backups.

We could fix this by

1. Making diff scan not return a DELETE for such a row
2. Implementing and using DELETE IGNORE in the restore job