Fix ignored sync work in passive effects

[gaearon]

Attempts to fix #14792. I don't really know this code but this is my guess.

Specifically, this code doesn't always end up being called from a root work cycle. It can run from the scheduler (indeed, that's the normal case). In that case isRendering is true inside (so ReactDOM.render doesn't do anything) but we don't flush anything before the exit. Now we do.

This delays the actual render until the end of calling effects. I think it makes sense as it's consistent with what we do for commit phase effects.

[sebmarkbage]

I'll let @acdlite review this one. Whenever he sees a bug like this, he finds two others.

[gaearon]

Fixed batching, good repro @sophiebits

[acdlite]

We do the same thing at the end of batchedUpdates et al. It's annoying, but will hopefully go away once we rely more on the Scheduler package (since Scheduler already does this check at the end of every task).