void poll(RequestRegistrationCallback<UndeleteOperation> requestRegistrationCallback) {
    cleanupExpiredInflightRequests(requestRegistrationCallback);
    checkAndMaybeComplete();
    if (!isOperationComplete()) {
      fetchRequests(requestRegistrationCallback);
    }
  }
Gets a list of {@link UndeleteRequest} for sending to replicas.
@param requestRegistrationCallback the {@link RequestRegistrationCallback} to call for every request
                           that gets created as part of this poll operation.
ambry-router/src/main/java/com/github/ambry/router/UndeleteOperation.java