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?
Ouch. :grimacing: This also assumes the servers are uniformly configured—the server with the fewest connections might not be able to accept any more (say, it's an `m3.medium` in a cluster with `m3.medium`s and `m3.large`s). I think we want to store percentages instead of raw connection counts.

Could we use round-robin to distribute clients among servers with the lowest percentages? Or randomly select a server from the list? Or assign weights to different servers, then cycle through them?