@Override
    public long getByteSize() {
        long size = 0;
        try (final ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
            try (final ObjectOutputStream oos = new ObjectOutputStream(baos)) {
                oos.writeObject(childMap);
            }
            size = baos.size();
        } catch (final IOException e) {
            LOGGER.debug(e::getMessage, e);
        }
        return size;
    }
Write data from the data store to an output removing them from the datastore as we go as they will be transferred
to another store.

@param output The output to write to.
stroom-query/stroom-query-common/src/main/java/stroom/query/common/v2/MapDataStore.java