private static String getXxhash64(List<String> lines) {
        return ConfigUtils.getXxhash64(new Utf8Array(Utf8.toBytes(skipEmptyLines(lines))));
    }
Computes xxhash64 of a list of strings. The only change to input lines before
computing xxhash64 is to skip empty lines.

@param lines A list of lines
@return the xxhash64 of the list, with lowercase letters
config/src/test/java/com/yahoo/vespa/config/util/ConfigUtilsTest.java