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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(['B', '1']);
    expect(ReactNoop.getChildren()).toEqual([span('B'), span('1')]);
    
    await advanceTimers(0);
    expect(ReactNoop.flush()).toEqual(['Promise resolved [2]']);
    expect(ReactNoop.getChildren()).toEqual([span('B'), span('1')]);
  });
  it('keeps working on lower priority work after being pinged', async () => {
    function App(props) {
      return (
        <Suspense fallback={<Text text="Loading..." />}>
          <AsyncText text="A" />
          {props.showB && <Text text="B" />}
        </Suspense>
      );
    }
    ReactNoop.render(<App showB={false} />);
    expect(ReactNoop.flush()).toEqual(['Suspend! [A]', 'Loading...']);
    expect(ReactNoop.getChildren()).toEqual([]);
    
    ReactNoop.expire(1200);
    ReactNoop.render(<App showB={true} />);
    expect(ReactNoop.flush()).toEqual(['Suspend! [A]', 'B', 'Loading...']);
    expect(ReactNoop.getChildren()).toEqual([]);
    await advanceTimers(0);
    expect(ReactNoop.flush()).toEqual(['Promise resolved [A]', 'A', 'B']);
    expect(ReactNoop.getChildren()).toEqual([span('A'), span('B')]);
  });
  it('tries rendering a lower priority pending update even if a higher priority one suspends', async () => {
    function App(props) {