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.
Since CacheNode is now a data container, it also records which params its
data was rendered under: `varyParams` is the source of the params `rsc`
depends on, carried from the decoded response (as a settled thenable that
`readVaryParams` reads) onto the node and into its BFCache entry, and
filled in alongside `rsc` when a deferred response arrives. Nothing reads
it yet; it's here so the node describes its own data.
Andrew Clark
d91019157b