[Segment Cache] Background segment revalidation (#74057)
This implements background revalidation of partial segment entries in
the client Segment Cache.
Until this PR, entries were never replaced in the Segment Cache, they
were only evicted upon becoming stale, or if the LRU overflowed. But
there are cases where we'll want to replace an existing entry with a new
one; for example, if the current segment has more dynamic holes than the
new one.
Most commonly this happens when multiple fetching strategies are mixed
within the same app. For example: when a shared layout belongs to both a
PPR-enabled route and a non-PPR enabled route, the layout data might be
omitted when prefetching the non-PPR enabled route, if it's wrapped in a
loading boundary. But that shouldn't prevent it from being included when
prefetching the route that has PPR enabled.
(*Another example is when a shared layout is prefetched both on viewport
entry and also via `<Link prefetch={true}>`. A viewport prefetch will
only include the static data, but `prefetch={true}` includes both the
static and the dynamic.*
*`<Link prefetch={true}>` is not yet implemented by the Segment Cache
but it will use a similar background revalidation strategy as the one
implemented in this PR.*)
