Use pipeline with multi-exec to set multiple keys at once

[boesing]

Feature Request

Q	A
New Feature	yes
BC Break	no

Summary

As of now, RedisCluster uses setex for every item and therefore does not persist all the keys at once. This might be inefficient when it comes to a huge amount of keys.

I am following redis/redis#5867 for quite a while and there was some frequent update:
redis/redis#5867 (comment)

So the user @oranagra states that using a pipeline along with multi-exec might be as performant as using MSETEX.

This issue is meant to be an idea of how to further optimize StorageInterface#setItems for redis cluster.

[boesing]

I've had a quick glance but it seems that using multi across cluster does not work as expected.
Got an exception:

RedisClusterException: Error processing EXEC across the cluster

Ref: phpredis/phpredis#868