Added hiredis like response handling

Added a couple of simple mechanisms that parse Redis responses
in a hiredis-like way, where replies are returned in clusterReply
structures, and can handle N level deep nesting, etc
Use CLUSTER SLOTS to get node information

Use the new command by @mattsta to map the keyspace on construction
rather than CLUSTER NODES.  Output from CLUSTER SLOTS is much easier
to process, and has everything we need.