Map<DataNodeId, List<ExchangeMetadataResponse>> getExchangeMetadataResponsesInEachCycle() {
    return exchangeMetadataResponsesInEachCycle;
  }
This is only used in test cases.
@return A map from DataNodeId to the list of ExchangeMetadataResponse in one replication cycle.
ambry-replication/src/main/java/com/github/ambry/replication/ReplicaThread.java