public synchronized List<DATATYPE> drain() {
        List<DATATYPE> dataListCopy = new ArrayList<>(dataList);
        dataList.clear();
        return dataListCopy;
    }
Gets and removes all the data currently available in this.
The returned list is a modifiable fresh instance owned by the caller.
container-core/src/main/java/com/yahoo/processing/response/DefaultIncomingData.java