etcd: Add world API changes for watching member status

This adds some useful functionality so that anyone with access to the
world API, can learn information about the changing etcd cluster it's
using.

etcd/world.go

@@ -211,3 +211,8 @@ func (obj *World) Fs(uri string) (engine.Fs, error) {
 	}
 	return etcdFs, nil
 }
+
+// WatchMembers returns a channel of changing members in the cluster.
+func (obj *World) WatchMembers(ctx context.Context) (<-chan *interfaces.MembersResult, error) {
+	return obj.Client.WatchMembers(ctx)
+}