Bug: Normal update between Idle render and a Ping causes Fallback to get stuck

[gaearon]

Repro case with a master build: https://codesandbox.io/s/stoic-mcnulty-dhygf?file=/src/App.js:668-703

Expected: we see content after a second.
Actual: fallback never resolves.

This happens in a sequence of:

1. Normal update
   • which suspends
2. Idle update
   • which also suspends
3. An unrelated normal update immediately followed by a ping (so they're batched)
   • here, we decide to stay on fallback, but should've shown the content

If you remove _setVersion(v => v + 1); on line 44 then the issue goes away.