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
+	err = i.Stop()
+	if err != nil {
+		return i, err
+	}
 	for _, shutdownFunc := range i.onShutdown {
 		err = shutdownFunc()
 		if err != nil {
 			return i, err
 		}
 	}
-	err = i.Stop()
-	if err != nil {
-		return i, err
-	}
 
 	// Execute instantiation events
 	EmitEvent(InstanceStartupEvent, newInst)