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**Do you want to request a *feature* or report a *bug*?**
Not sure if its a bug or a feature request.

**What is the current behavior?**
The component given as a fallback to Suspense is remounted each time a lazy child resolves.

https://codesandbox.io/s/z6v6x3n1np
This example shows how a fallback component, which counts up, is mounted more than once.

**What is the expected behavior?**
The fallback is mounted once and is only unmounted when all lazy children are resolved.
In the example the counter would run up to 9.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
react@16.6.0

await advanceTimers(50);
    
    expect(ReactNoop.flush()).toEqual([
      'Promise resolved [A]',
      'Foo',
      'Bar',
      'A',
      'B',
    ]);
    expect(ReactNoop.getChildren()).toEqual([span('A'), span('B')]);
  });
  it('suspends siblings and later recovers each independently', async () => {
    
    ReactNoop.render(
      <Fragment>
        <Suspense maxDuration={1000} fallback={<Text text="Loading A..." />}>
          <AsyncText text="A" ms={5000} />
        </Suspense>
        <Suspense maxDuration={3000} fallback={<Text text="Loading B..." />}>
          <AsyncText text="B" ms={6000} />
        </Suspense>
      </Fragment>,
    );
    expect(ReactNoop.flush()).toEqual([
      'Suspend! [A]',
      'Loading A...',
      'Suspend! [B]',
      'Loading B...',
    ]);
    expect(ReactNoop.getChildren()).toEqual([]);