@Override
        public long count() {
            return countChildren(
                    key,
                    key.getChildDepth());
        }
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/LmdbDataStore.java