Use RouteTree<T> for CacheNode tree
Refactors the CacheNode tree to use the RouteTree<T> data type. CacheNode used to be a tree-shaped object, with a similar structure to RouteTree<T>. CacheNode is now a data container instead, embedded inside a RouteTree structure.
So the usage sites change from CacheNode to RouteTree<CacheNode>.
As a result, we can now diff against the render tree directly during prefetches and navigations, instead of diffing against FlightRouterState. This gets us closer to migrating everything away from FlightRouterState to our shared data types.
