@Override
        public String toString() {
            return "ReferenceLoader{" +
                   "loaderPipeline=" + loaderPipeline +
                   ", referenceFeed=" + referenceFeed +
                   ", streamType='" + streamType + '\'' +
                   '}';
        }
@return data string or epoch ms
stroom-pipeline/src/main/java/stroom/pipeline/refdata/RefDataLookupRequest.java