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?
It feels odd to me that there appears to be initialization of this list occurring at the same time the list is being traversed. I presume this logic is meant such that any node with child nodes is skipped, but all the children are added (where they to will then be skipped if they also have child nodes). This would be clear if the creation of the list was a distinct step separate from traversing the list. I'm very wary of logic that manipulates a list being traversed *as it's being traversed*, its prone to error.