serving: Brief 404 errors after creating a route.

In what area(s)?

/area API /area networking

What version of Knative?

HEAD

Expected Behavior

When a route/kservice is Ready, I expect it to be routable immediately without issues.

Actual Behavior

There is a brief duration of 404s before the networking state is propagated to the routers and is consistent.

Steps to Reproduce the Problem

A reproducer is supposed to be produced by #3287