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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

expect(ReactTestRenderer).toHaveYielded(['Loading...']);
    expect(root).toMatchRenderedOutput('Loading...');
    await LazyClass;
    expect(ReactTestRenderer).toHaveYielded(['Hi', 'Did mount: Hi']);
    expect(root).toMatchRenderedOutput('Hi');
  });
  it('only captures if `fallback` is defined', () => {
    const root = ReactTestRenderer.create(
      <Suspense fallback={<Text text="Loading..." />}>
        <Suspense maxDuration={100}>
          <AsyncText text="Hi" ms={5000} />
        </Suspense>
      </Suspense>,
      {
        unstable_isConcurrent: true,
      },
    );
    expect(root).toFlushAndYield([
      'Suspend! [Hi]',
      
      
      'Loading...',
    ]);
    jest.advanceTimersByTime(1000);
    expect(ReactTestRenderer).toHaveYielded([]);
    expect(root).toFlushAndYield([]);
    expect(root).toMatchRenderedOutput('Loading...');
    jest.advanceTimersByTime(5000);
    expect(ReactTestRenderer).toHaveYielded(['Promise resolved [Hi]']);
    expect(root).toFlushAndYield(['Hi']);