Continue page rendering after handling server requests (#47089)
This moves the position of action handling so it can continue page
rendering afterwards. And a `asNotFound` option is added to
`LayoutRouter` to indicate the router to opt into the root not-found
boundary, so not found pages can be programmatically rendered instead of
relying on route mismatching (this is also needed by NEXT-463 later).
Fixes NEXT-467.
