[FIX] component: async issue

This is a tricky commit. The key point is that the Fiber.complete
method, which commits a rendering to the DOM works like this: it
traverses the component tree, patch the corresponding DOM for each
component, calls the mounted/destroy hooks, and reset the currentfiber
of components to null, all synchronously.

However, this means that it is possible for components to initiate a
rendering (which create a new currentFiber) before the currentFiber is
reset to null, so the internal state of owl is corrupted. This can
occurs in a crash, as in the test that accompanies this commit.

To fix this, we take care of resetting the currentFiber first, while we
walk the component tree. Then, the internal state is always consistent
(i.e. a currentFiber to null means that there is no pending rendering)

closes #904

src/component/component.ts

@@ -400,6 +400,7 @@ export class Component<Props extends {} = any, T extends Env = Env> {
     if (currentFiber && !currentFiber.isRendered && !currentFiber.isCompleted) {
       return scheduler.addFiber(currentFiber.root);
     }
+
     // if we aren't mounted at this point, it implies that there is a
     // currentFiber that is already rendered (isRendered is true), so we are
     // about to be mounted
@@ -505,7 +506,6 @@ export class Component<Props extends {} = any, T extends Env = Env> {
     const __owl__ = this.__owl__;
 
     __owl__.status = STATUS.MOUNTED;
-    __owl__.currentFiber = null;
     this.mounted();
     if (__owl__.mountedCB) {
       __owl__.mountedCB();

src/component/fiber.ts

@@ -198,6 +198,7 @@ export class Fiber {
     // build patchQueue
     const patchQueue: Fiber[] = [];
     const doWork: (Fiber) => Fiber | null = function (f) {
+      f.component.__owl__.currentFiber = null;
       patchQueue.push(f);
       return f.child;
     };
@@ -255,10 +256,6 @@ export class Fiber {
           component.__owl__.pvnode!.elm = component.__owl__.vnode!.elm;
         }
       }
-      const compOwl = component.__owl__;
-      if (fiber === compOwl.currentFiber) {
-        compOwl.currentFiber = null;
-      }
     }
 
     // insert into the DOM (mount case)