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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

<Yield renderTime={5} />
              <Yield renderTime={10} />
              <React.unstable_Profiler id="inner" onRender={callback}>
                <Yield renderTime={17} />
              </React.unstable_Profiler>
            </React.unstable_Profiler>,
            {unstable_isConcurrent: true},
          );
          expect(renderer).toFlushAndYieldThrough(['Yield:5']);
          expect(callback).toHaveBeenCalledTimes(0);
          
          advanceTimeBy(50); 
          
          
          expect(renderer).toFlushAndYield(['Yield:10', 'Yield:17']);
          expect(callback).toHaveBeenCalledTimes(2);
          const [innerCall, outerCall] = callback.mock.calls;
          
          
          expect(innerCall[0]).toBe('inner');
          expect(innerCall[2]).toBe(17); 
          expect(innerCall[3]).toBe(17); 
          expect(innerCall[4]).toBe(70); 
          expect(innerCall[5]).toBe(87); 
          expect(outerCall[0]).toBe('outer');
          expect(outerCall[2]).toBe(32); 
          expect(outerCall[3]).toBe(32); 
          expect(outerCall[4]).toBe(5); 
          expect(outerCall[5]).toBe(87); 
        });