public Map<String, Set<ReplicaId>> getLeaderPartitionToPeerLeaderReplicas() {
      return Collections.unmodifiableMap(leaderPartitionToPeerLeaderReplicas);
    }
Get a map of partitions to their sets of peer leader replicas (this method is only by ReplicationTest for now)
@return an unmodifiable map of peer leader replicas stored by partition
ambry-replication/src/main/java/com/github/ambry/replication/ReplicationEngine.java