Improve cluster mapping during client init

[mellis13]

Description

Currently RedisCluster::_map_custer uses the redis++ command function to execute CLUSTER SLOTS for cluster mapping. However, this does not necessarily leverage the use of an existing connection and all clients in an MPI simulation may be directed at the same node to execute CLUSTER SLOTS. Client init performance should improve if new Command functionality is leveraged to address the CLUSTER SLOTS command at the same node initially connected.

Justification

Users should see improved clinit init performance at large scales.

Implementation Strategy

The new Command functionality that is being implemented in #113 should be leveraged to address the CLUSTER SLOTS command at the node that was used for initial connection. Another path that should be briefly looked at is to see if the redis++ client exposes any methods for retrieving cluster slots information so that the redundant CLUSTER SLOTS call isn't needed.