Redis Cluster: Cluster ID / Name

[dw130]

Hi:
I face a problem which desc in #2472

It is truly dangerous when a company own many clusters and all ips are visible,because some somebody else may join many redis cluster together intentionally or unintentionally,It may be
an very serious problem

So I add a cluster-name testaa in redis.conf, hash the cluster name string into a int flag,
check the int flag when counter a CLUSTERMSG_TYPE_MEET  message between cluster nodes.

This problem is desc in #2472 as an proposed-feature feature,
But I really need this feature, could you please take a look at my-code and check as it is right as I am not quite sure. Thanks very much

[CLAassistant]

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