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?
`fetchLoopButNoNotThatLoop`? :wink: I could see this happening, actually. `pollCounts` (and `publishCounts`) would be clearer.