@Override
  protected boolean isContinuousReplicationEnabled(ReplicationConfig replicationConfig) {
    return false;
  }
This class extends the replication logic encapsulated in ReplicaThread. Instead of apply updates
from various replicas, it checks if the blobs are already in the updated state in local store.
If the local store was recovered from a backup, then we are effectively checking the blob state in backup.
The backup is a union of all on-prem replicas. So the latest state of the blob should be reflected in the backup.
There are two possibilities:
1> The backup may be lagging behind the replica and if we recover from such a lagging backup, then we'd be recovering
to a point in time in the past.
2> The backup may be ahead of the replica. This can happen if the replica is lagging behind its peers. If we recover
from such a backup, we'd still be consistent from the user's point of view. The lagging replica must catch up with
its peers and this checker will detect such lagging replicas as well.
ambry-replication/src/main/java/com/github/ambry/replication/BackupCheckerThread.java