Make Router faster to clone

[davidpdrsn]

Motivation

Router is cloned once for every request in IntoMakeService::call so ideally it should be cheap to do so.

Solution

Wrap the Node in a Arc and rebuild it when calling Router::router. This is fine since it only happens during startup.

I did a simple benchmark where I cloned a Router with 1000 routes and this brought the time down from 176.916µs to 92.698µs. So almost 50% 📈

I also attempted to Arc the other fields but since they're !Sync the Arc becomes !Send which is a no-go. Using SyncWrapper doesn't help as that requires exclusive access which we cannot get through an Arc.