Open a single control connection per cluster

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All other C* drivers built by Datastax (for Python, PHP, Java, Ruby, Node) to this and are successful 😄

I asked about this in Datastax's forums and they confirmed that this is a good idea.

This should drastically simplify Xandra.Cluster's code. We could probably embed the single control connection straight into the Xandra.Cluster process and avoid the child control connection process altogether.