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?
This is probably the wrong thing to do. I can just imagine one server having the lowest, and 15+ other servers all telling everyone to go hit that one. Then that one poor server gets bogged down and fails to update etcd quickly enough before the rest go and tell another swarm of ppl to hit the min peer...