Fix: Synchronous popstate transitions

This is a refactor of the fix in facebook#27505.

When a transition update is scheduled by a popstate event, (i.e. a back/
forward navigation) we attempt to render it synchronously even though
it's a transition, since it's likely the previous page's data is cached.

In facebook#27505, I changed the implementation so that it only "upgrades" the
priority of the transition for a single attempt. If the attempt
suspends, say because the data is not cached after all, from then on it
should be treated as a normal transition.

But it turns out facebook#27505 did not work as intended, because it relied on
marking the root with pending synchronous work (root.pendingLanes),
which was never cleared until the popstate update completed.

The test scenarios I wrote accidentally worked for a different reason
related to suspending the work loop, which I'm currently in the middle
of refactoring.

Author: acdlite