Router: Use a goroutine pool for routing messages.

* Add `RouterConfig.PoolSize`.
* Use newline-delimited strings instead of JSON for router messages.
* Add `routablePool` and `Routable.Recycle()`.
`EtcdLocator`: Store full server URLs in etcd.

* Remove `RouterConfig.{Scheme, UrlTemplate}`.
* Add `Router.URL()`.
* Remove `Router.formatURL()`.
* Update the sample config.