[Segment Cache] Prioritize route trees over segments (#75213)
When processing the prefetch queue, we should prioritize fetching the
route trees for all the links before we fetch any of the segments.
The reason the route tree is more important is because it tells us the
structure of the target page; from this alone, we can determine which
segments are already cached and skip them during the navigation. Whereas
if the route tree is missing at the time of the navigation, we cannot
instruct the server to skip over prefetched segments, because we don't
know which ones to skip.
To implement this, I added a `phase` property to the PrefetchTask type.
This is essentially a secondary priority field that changes as the task
progresses. There are two phases: `RouteTree` and `Segments`. Any task
in the `RouteTree` phase has higher priority than tasks in the
`Segments` phase.
Another way to implement this would be to add a secondary queue for
segment prefetches. This would require reference counting to clean up
segments when there are no more tasks that depend on them. The ref count
could also be used as an additional prioritization heuristic: shared
layouts with many dependent tasks could be prioritized over segments
with fewer. However, I'm not sure the benefits are worth the extra
code/complexity, so I'm starting with this simpler approach.
