private Long getLong(final ByteBuffer valueBuffer, final int index) {
        final long l = valueBuffer.getLong(index);
        if (l == -1) {
            return null;
        }
        return l;
    }
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