Use `NodeCounters` `NodeId`s as the blinded path intro references

Use `NodeCounters` `NodeId`s as the blinded path intro references

The router's `introduction_node_id_cache` is used to cache the
`NodeId`s of blinded path introduction points so that we don't
have to look them up every time we go around the main router loop.
When using it, if the introduction point isn't a public node we
then look up the introduction in our first-hops map.

In either case, we have to end up with a reference to a `NodeId`
that outlives our `dist` map.

Here we consolidate both the initial cache building and the
first-hops map lookup to one place, storing only a reference to a
`NodeId` either in the `NetworkGraph` or in the new `NodeCounters`
to get the required lifetime without needing to reference into the
first-hops map.

We then take this opportunity to avoid `clone`ing the first-hops
map entries as we now no longer reference into it.