RedisRDD/groupKeysByNode: return Array instead of Map
RedisRDD/groupKeysByNode: return Array instead of Map

We may get many ZSets at a time, ZSetRDD can consists of
duplicated keys

Helped-by: Itamar Harber <6906be8b99a659e743674ece90805ac5611ad258@redislabs.com>

RedisLabs/spark-redis,RedisLabs/spark-redis