dillon1000/react

Prepare placeholders before timing out (#13092)

* Prepare placeholders before timing out

While a tree is suspended, prepare for the timeout by pre-rendering the
placeholder state.

This simplifies the implementation a bit because every render now
results in a completed tree.

* Suspend inside an already timed out Placeholder

A component should be able to suspend inside an already timed out
placeholder. The time at which the placeholder committed is used as 
the start time for a subsequent suspend.

So, if a placeholder times out after 3 seconds, and an inner
placeholder has a threshold of 2 seconds, the inner placeholder will
not time out until 5 seconds total have elapsed.