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?
The lock protects `b.{fetchErr, lastFetch, peerURL, peerConns}`. If `err != nil`, we set `b.fetchErr`; otherwise, `b.{lastFetch, peerURL, peerConns}`. It's needed for both, no?