[Fix] Propagate redis_timeout into nested greylist.redis{} block

[Fix] Propagate redis_timeout into nested greylist.redis{} block

When Redis is configured via greylist { redis { ... } }, parse_redis_server
reads opts.redis and ignores opts.timeout, so redis_timeout had no effect.
Shallow-copy the nested redis subtable and set its timeout when not
explicitly configured by the operator.