[Fizz] Simplify ReplayNode data structure (facebook#27395)

The key is that instead of storing different tags of resumable points,
we just store if a replay node has any resumable slots and if that's at
the root `number` or if it has resumable slots by index.

This is a simpler and more compact format because we don't have to
separate the three Resume forms.

This helps deal with Postpone in fallbacks because it doesn't just
double all the cases.