etcd: Rewrite embed etcd implementation

This is a giant cleanup of the etcd code. The earlier version was
written when I was less experienced with golang.

This is still not perfect, and does contain some races, but at least
it's a decent base to start from. The automatic elastic clustering
should be considered an experimental feature. If you need a more
battle-tested cluster, then you should manage etcd manually and point
mgmt at your existing cluster.

util/sync_test.go

@@ -86,7 +86,7 @@ func TestEasyAckOnce2(t *testing.T) {
 	}
 }
 
-func ExampleSubscribeSync() {
+func ExampleSubscribedSignal() {
 	fmt.Println("hello")
 
 	x := &SubscribedSignal{}