ns_server may lose replicas on stopped rebalance/graceful failover (was: {DCP} : Delta Recovery Impossible after re-try of graceful failover since in first attempt failed)

Scenario
1. Create a 7 Node cluster
2. Create default bucket with 100 K items
3. Graceful failover a node
4. Kill memcached of another node during graceful failover
5. Graceful failover the same node in step 3
6. Add-back the node with delta recovery
7. Hit Rebalance

In Step 7, Rebalance fails for delta recovery. Says delta recovery is not possible. Although we see nodes in the cluster are in healthy state.

We see the following warning:: "Fail Over Warning: Rebalance required, some data is not currently replicated!”

Seems like the delta recovery will not work in this condition, unless we rebalance the cluster. Also, I was able to cancel the delta recovery and do full recovery.

Opening the bug to follow-up on the issue. Attaching logs and data files