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?
TODO: Future conflict. This may need to be patched to Statistician, if landing after #135. Other incidents of it would be auto-patched.
![tumblr_mxq826laoe1rawb5do1_400](https://cloud.githubusercontent.com/assets/35755/5149766/d91cddf4-7180-11e4-9d8f-c718b0a1fc7d.jpg)