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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(getWorkForReactThreads(onWorkStarted)).toHaveLength(0);
      expect(getWorkForReactThreads(onWorkStopped)).toHaveLength(0);
      onRender.mockClear();
      advanceTimeBy(3);
      
      const interactionTwo = {
        id: 2,
        name: 'root update event',
        timestamp: mockNow(),
      };
      SchedulerTracing.unstable_trace(interactionTwo.name, mockNow(), () => {
        renderer.update(
          <React.unstable_Profiler id="test-profiler" onRender={onRender}>
            <Example />
          </React.unstable_Profiler>,
        );
      });
      expect(onInteractionTraced).toHaveBeenCalledTimes(3);
      expect(onInteractionTraced).toHaveBeenLastNotifiedOfInteraction(
        interactionTwo,
      );
      expect(onInteractionScheduledWorkCompleted).toHaveBeenCalledTimes(2);
      
      
      expect(getWorkForReactThreads(onWorkStarted)).toHaveLength(0);
      expect(getWorkForReactThreads(onWorkStopped)).toHaveLength(0);
      
      
      
      expect(onWorkScheduled).toHaveBeenCalled();