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?
For reasons I mention below, I think its kind of scary to have every node in the cluster returning the same single node to redirect people to. If all 10 servers are within 10% of each other connection count-wise, one is gonna get slammed unfairly. This should be replaced with an algorithm that returns a server randomly from its peers weighted on connections-free.