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?
ok, this is probably silly, but I'm wondering if using "loop" here might cause confusion in the future. ("is this just for Hello/Loop?")

I've seen that kind of unsmart thinking in the past.