split or merge removed region under snapshot

We run into snapshot failures for one table with large number of regions. The event sequence is like the following:

1. Snapshot process lists all regions for one table.
2. Normalize kicks in to split some regions for the table under snapshot.
3. split finishes and major compaction finishes. The parent region is moved to archive.
4. When the Snapshot processes the parent region, it does not exist and snapshot fails.

Since snapshot process acquires the table lock, but there is no table lock acquired in split or merge process, they crash into each other.