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?
We needed a value that's less than the absolute maximum number of connected clients, to allow for some overage. 0.75 was arbitrarily chosen (but allowed to be overridden by config)  because some value had to be picked. It could have been 0.99, but it's hard to determine that with little to no load testing.