Alert if KV replica is behind indexer for long time

Indexer has a mechanism to detect when KV replica is behind index persisted sequence number. This allows indexer to retain extra disk snasphots for better resilience in case of a failover and avoid rollback to 0. However, indexer only retains 2 extra disk snapshots by default in addition to 2 regular disk snasphots. This means that if KV replica is behind for more than 40mins, there can will be rollback to 0, in case of a failover.

Indexer should alert once it detects that KV replica is behind for more than 40mins and a failover would lead to rollback to 0.