fix(reactivity): ensure computed always expose value

fix #3099

Also changes the original fix for #910 by moving the fix from
reactivity to the scheduler

packages/reactivity/__tests__/effect.spec.ts

@@ -709,28 +709,6 @@ describe('reactivity/effect', () => {
     expect(dummy).toBe(3)
   })
 
-  it('stop with scheduler', () => {
-    let dummy
-    const obj = reactive({ prop: 1 })
-    const queue: (() => void)[] = []
-    const runner = effect(
-      () => {
-        dummy = obj.prop
-      },
-      {
-        scheduler: e => queue.push(e)
-      }
-    )
-    obj.prop = 2
-    expect(dummy).toBe(1)
-    expect(queue.length).toBe(1)
-    stop(runner)
-
-    // a scheduled effect should not execute anymore after stopped
-    queue.forEach(e => e())
-    expect(dummy).toBe(1)
-  })
-
   it('events: onStop', () => {
     const onStop = jest.fn()
     const runner = effect(() => {}, {