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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

renderer.unstable_flushAll(['FirstComponent']);
        expect(onRender).toHaveBeenCalledTimes(1);
        call = onRender.mock.calls[0];
        expect(call[0]).toEqual('test');
        expect(call[5]).toEqual(mockNow());
        expect(call[6]).toMatchInteractions(
          ReactFeatureFlags.enableSchedulerTracing ? [interactionLowPri] : [],
        );
        expect(onInteractionTraced).toHaveBeenCalledTimes(2);
        expect(onInteractionScheduledWorkCompleted).toHaveBeenCalledTimes(1);
        
        
        expect(getWorkForReactThreads(onWorkStarted)).toHaveLength(3);
        expect(getWorkForReactThreads(onWorkStarted)[1][0]).toMatchInteractions(
          [interactionLowPri, interactionHighPri],
        );
        expect(getWorkForReactThreads(onWorkStarted)[2][0]).toMatchInteractions(
          [interactionLowPri],
        );
        expect(getWorkForReactThreads(onWorkStopped)).toHaveLength(2);
        expect(getWorkForReactThreads(onWorkStopped)[0][0]).toMatchInteractions(
          [interactionLowPri, interactionHighPri],
        );
        expect(getWorkForReactThreads(onWorkStopped)[1][0]).toMatchInteractions(
          [interactionLowPri],
        );
      });
      expect(onInteractionTraced).toHaveBeenCalledTimes(2);