Support redis 6 ACLs

[laurencegill]

Make ACLs work with redis 6, should support the default user for <6 versions:

In the default configuration, Redis 6 (the first version to have ACLs) works exactly like older versions of Redis, that is, every new connection is capable of calling every possible command and accessing every key, so the ACL feature is backward compatible with old clients and applications. Also the old way to configure a password, using the requirepass configuration directive, still works as expected, but now what it does is just to set a password for the default user.