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/interfaces/client.go

@@ -60,4 +60,7 @@ type Client interface {
 	Txn(ctx context.Context, ifCmps []etcd.Cmp, thenOps, elseOps []etcd.Op) (*etcd.TxnResponse, error)
 	Watcher(ctx context.Context, path string, opts ...etcd.OpOption) (chan error, error)
 	ComplexWatcher(ctx context.Context, path string, opts ...etcd.OpOption) (*WatcherInfo, error)
+
+	// WatchMembers returns a channel of changing members in the cluster.
+	WatchMembers(context.Context) (<-chan *MembersResult, error)
 }