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?
Well, since 0.75 had little reason behind it, I'm perfectly fine boosting this to 0.95, particularly since we can still do fine grain adjustments later.