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?
Maybe an example of what exactly this etcd key looks like? I have a tough time figuring out if the code is correct since I don't know offhand what the format of this key is.