Store client counts in etcd

This pull request adds an `EtcdBalancer` that keeps track of client counts for different hosts, and can redirect clients to different hosts if necessary. The redirect support has been temporarily removed.

@bbangert, @jrconlin r?
What is root? Is it the root directory where all the clusters register under their own name? If its the directory that this cluster registers under than none of the nodes will have child nodes, no?