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?
Ah, ok, I was actually thinking of something like this before. I was thinking leaving maybe 5% of max connections free to handle redirecting pp (ie 0.95), since 0.75 feels like a lot of reserved capacity just to tell ppl to go away.