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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(ReactNoop.flush()).toEqual(['Suspend! [A]', 'Loading...']);
    expect(ReactNoop.getChildren()).toEqual([]);
    
    
    await advanceTimers(4999);
    ReactNoop.expire(4999);
    expect(ReactNoop.flush()).toEqual([]);
    expect(ReactNoop.getChildren()).toEqual([]);
    
    ReactNoop.render(<App text="B" />);
    
    expect(ReactNoop.flush()).toEqual(['Suspend! [B]', 'Loading...']);
    expect(ReactNoop.getChildren()).toEqual([]);
    
    
    ReactNoop.interactiveUpdates(() => {
      ReactNoop.render(<App text="C" />);
    });
    expect(ReactNoop.flush()).toEqual(['C']);
    expect(ReactNoop.getChildren()).toEqual([span('C')]);
    await advanceTimers(10000);
    
    expect(ReactNoop.flush()).toEqual([
      'Promise resolved [A]',
      'Promise resolved [B]',
    ]);
    expect(ReactNoop.getChildren()).toEqual([span('C')]);
  });
  it('flushes all expired updates in a single batch', async () => {
    class Foo extends React.Component {