embed: Move the ReadyNotify() call to a better place

When using the embed functionality, you can't call the Server.Stop()
function until StartEtcd returns, which can block until there is a call
to Server.Stop() in error situations. Since we have a catch-22, the
ReadyNotify() can be called manually by the user if they wish to wait
for the server startup, or in parallel with a timeout if they wish to
cancel it after some time.

Chzz pointed out that this is also more consistent with the
etcdserver.Start() behaviour too.

purpleidea pointed out that this is actually more correct too, because
we can now register the stop interrupt handler before we block on
startup.

integration/embed_test.go

@@ -65,6 +65,9 @@ func TestEmbedEtcd(t *testing.T) {
 	for i, tt := range tests {
 		tests[i].cfg.Dir = dir
 		e, err := embed.StartEtcd(&tests[i].cfg)
+		if e != nil {
+			<-e.Server.ReadyNotify() // wait for e.Server to join the cluster
+		}
 		if tt.werr != "" {
 			if err == nil || !strings.Contains(err.Error(), tt.werr) {
 				t.Errorf("%d: expected error with %q, got %v", i, tt.werr, err)