[FIX] component: concurrent calls to mount and render

Closes #450

src/hooks.ts

@@ -24,9 +24,19 @@ import { Observer } from "./core/observer";
 export function useState<T>(state: T): T {
   const component: Component<any, any> = Component.current!;
   const __owl__ = component.__owl__;
+  const renderFn = __owl__.renderFn;
+  let lastRenderRevNumber;
+  __owl__.renderFn = function(comp, params) {
+    lastRenderRevNumber = __owl__.observer!.rev;
+    return renderFn(comp, params);
+  };
   if (!__owl__.observer) {
     __owl__.observer = new Observer();
-    __owl__.observer.notifyCB = component.render.bind(component);
+    __owl__.observer.notifyCB = () => {
+      if (lastRenderRevNumber < __owl__.observer!.rev) {
+        component.render();
+      }
+    };
   }
   return __owl__.observer.observe(state);
 }