Redis Cluster Support

[kranikitao]

Need to add redis cluster support to the client library.

Redis Cluster Specification - https://redis.io/topics/cluster-spec

[kelunik]

The major difficulty will be extracting keys from the commands, because the QueryExecutor currently only gets an array of arguments, but doesn't know which ones are keys and which aren't. Knowning the keys is, however, important to use a client side routing table. Without a client side routing table, most queries will have to be redirected by the first server being queried, which is inefficient.