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?
Keep in mind with these slicing operations, if by any chance one of them is the last char in the string, you slice on it + 1... which would panic. Granted we *hope* the scheme is a string in the right format, but if it gets set incorrectly for some reason than this type of slicing is prone to a panic.