Snapshot purge should be a atomic operation

After HDDS-8665, there is a possibility that snapshot cache gets updated but snapshot chain update fails and leave it in a situation that snapshot's previous snapshot is pointing to something which doesn't exist or order is messed.

We need to revisit this and see if there is any race condition issue in snapshot purge.

One possible solution is, snapshot purge should be single snapshot purge request instead of batch request.
Other thing is we may need locking in snapshot purge request handler.