void registerRequestToSend(T routerOperation, RequestInfo requestInfo) {
    // TODO: netty send here>?
    if (requestsToSend != null) {
      requestsToSend.add(requestInfo);
    }
    correlationIdToOperation.put(requestInfo.getRequest().getCorrelationId(), routerOperation);
  }
@param routerOperation the operation that the request corresponds to.
@param requestInfo the request to send out.
ambry-router/src/main/java/com/github/ambry/router/RequestRegistrationCallback.java