Add support of Redis JSON

Hi, As Redis is now supporting JSON, it will be great to also store the session data directly in this format. This will allows for example to create indexes and find sessions easily against some details as user id or whatever you want. And this is much more optimized than SCAN/MGET over a prefix. So I propose a small enhancement to add this option (useRedisJson) which is disabled by default to keep standard behavior if not configured. I've also reworked the build workflow to run test over redis-stack-server under Ubuntu 20.04 which support it via the redis repo. Hope it will help... Regards,