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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

ReactNoop.expire(4000);
    await advanceTimers(4000);
    expect(ReactNoop.flush()).toEqual([]);
    expect(ReactNoop.getChildren()).toEqual([
      span('Loading A...'),
      span('Loading B...'),
    ]);
    
    
    
    ReactNoop.expire(1000);
    await advanceTimers(1000);
    expect(ReactNoop.flush()).toEqual(['Promise resolved [A]', 'A']);
    expect(ReactNoop.getChildren()).toEqual([span('A'), span('Loading B...')]);
    
    
    ReactNoop.expire(1000);
    await advanceTimers(1000);
    expect(ReactNoop.flush()).toEqual(['Promise resolved [B]', 'B']);
    expect(ReactNoop.getChildren()).toEqual([span('A'), span('B')]);
  });
  it('continues rendering siblings after suspending', async () => {
    ReactNoop.render(
      <Suspense fallback={<Text text="Loading..." />}>
        <Text text="A" />
        <AsyncText text="B" />
        <Text text="C" />
        <Text text="D" />
      </Suspense>,