caddy: Run OnShutdown callbacks before instance Stop() calls (#2320)

- see https://github.com/coredns/coredns/issues/1666#issuecomment-380624422

caddy.go

@@ -248,16 +248,16 @@ func (i *Instance) Restart(newCaddyfile Input) (*Instance, error) {
 	}
 
 	// success! stop the old instance
-	for _, shutdownFunc := range i.onShutdown {
-		err = shutdownFunc()
+	err = i.Stop()
 	if err != nil {
 		return i, err
 	}
-	}
-	err = i.Stop()
+	for _, shutdownFunc := range i.onShutdown {
+		err = shutdownFunc()
 		if err != nil {
 			return i, err
 		}
+	}
 
 	// Execute instantiation events
 	EmitEvent(InstanceStartupEvent, newInst)